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Troubleshooting an acceleration bog/hesitation |
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brhillman 
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Joined: August-01-2021 Location: Snohomish WA Status: Offline Points: 62 |
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 Topic: Troubleshooting an acceleration bog/hesitationPosted: August-03-2023 at 9:26pm |
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Yup, I’ve actually rebuilt it twice now! Couple more things to try. Still need to check if the secondaries are opening too soon and watch the pump shot under a hard pull with the flame arrestor off (need a good day with two adults and no children in the boat which is hard to come by at the moment). I hate throwing parts at a problem but getting to the point where it’d be nice to try a new carb with factory specs, at least to confirm I’m barking up the right tree and have a better idea of the baseline I’m starting with. Like I said the boat was not running great when I bought it and it’s tons better now but not much idea of what others have done to it before me. There were quite a bit of tool marks inside the carb from previous “rebuild” efforts from previous “mechanics”.
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91NaughtyQ
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Joined: July-27-2015 Location: NC Status: Offline Points: 289 |
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Posted: August-03-2023 at 8:59pm |
I am not near the expert others are but what you have done sounds right to me. Just putting this out there - have you done a rebuild on the carb?
Gary |
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1991 Ski Nautique
(Previous)1984 Ski Nautique 2001 |
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brhillman
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Posted: August-03-2023 at 2:48pm |
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Pump diaphragm is new, and I’ve had the issue with two different diaphragms in there. Could try replacing again though. Adjusted linkage for zero tolerance (with throttle at idle position), and verify first movement of throttle initiates the pump squirt. And I think the fact that I DO get an initial surge of acceleration immediately I think suggests that I am indeed getting a strong squirt off the start.
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91NaughtyQ
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Posted: August-03-2023 at 11:50am |
1 and 2 specifically 1 and making sure linkage adjusted properly has always fixed this issue when it pops up on my 91. It seems I get about 2 years out of the diaphragm before it looses some "spring".
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1991 Ski Nautique
(Previous)1984 Ski Nautique 2001 |
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GottaSki
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Joined: April-21-2005 Location: NE CT Status: Offline Points: 3363 |
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Posted: August-02-2023 at 8:41am |
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Yes
Seeing your PV is now 6.5 as recommended, go smaller ap nozzle, textbook for a longer duration shot
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"There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing."
River Rat to Mole |
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bwinn
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Joined: July-13-2009 Location: ct Status: Offline Points: 396 |
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Posted: August-01-2023 at 6:17pm |
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I’m no help but I gotta say, I don’t think I’ve ever seen a video embedded in a post like that. Kudos!
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brhillman
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Posted: August-01-2023 at 5:39pm |
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Also, the idea that maybe pump shot duration isn’t long enough has me wondering if maybe I should try a SMALLER pump nozzle. The engine absolutely had a vacuum leak when I bought it, which I fixed (carb spacer leaking when engine would warm up), so I wonder if someone tried to drill out the pump nozzle at some point to compensate for the extra air getting into the mixture. When I had last pulled the nozzle I didn’t have pin gauges to verify size, so this is an unknown variable I guess. It definitely wasn’t restricted.
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brhillman
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Posted: August-01-2023 at 5:35pm |
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A small update. First, I took some video that illustrates the issue. Showing RPM on a hard acceleration from idle. The quality of the video is terrible because of conversion from iPhone video to gif for hosting on imgbb to embed here, but you get the idea: RPM initially jumps as the engine gets that shot of fuel from the accelerator pump, then drops, then climbs back up and will continue to climb all the way up to top end without issue. My impression is that this might indicate that the duration of the pump shot isn’t long enough. I tried moving the pump cam from the #1 to the #2 position (and of course then readjusting the pump linkage to zero clearance). This didn’t have much impact, maybe a little bit better but that might just be wishful thinking. Any thoughts?
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KevinGodwin
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Joined: February-16-2020 Location: Toronto Status: Offline Points: 4 |
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Posted: July-20-2023 at 2:32pm |
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Hi BR, hope the new baby is doing well.
After cleaning the carb, which didn't help but was a good learning exercise, I changed the coil (used Flamethrower brand) and spark plugs and that seems to have done the trick! I am still having a little stuttering at the very bottom end but is getting better as I dial the carb in which has been harder than I thought it would. Fingers crossed it holds. I hope you get yours fixed. K |
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brhillman
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Posted: June-20-2023 at 12:48am |
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Thanks!
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KevinGodwin
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Posted: June-19-2023 at 7:26pm |
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Congrats on baby #2!
Coil was my next stop and another few cans of sea foam! Keep up the good fight. |
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brhillman
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Posted: June-19-2023 at 12:41am |
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Hey Kevin, no I haven’t we just had our second baby a couple weeks ago so haven’t had time to get the boat out more than once. I went back through the carb again before getting it back on the water this spring, and everything looked fine and seemed to be operating properly. Got it all back together, adjusted the timing, and took it out a month or so ago. Ran great other than that persistent hesitation under hard throttle. I too was starting to suspect something electrical or timing. It seemed to improve a bit when I changed the ignition coil last summer.
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KevinGodwin
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Posted: June-18-2023 at 9:32pm |
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Hi BR - any luck getting to the bottom of it? I thought my stuttering was gone but it came back today. It’s been running great through May and early June, but today was warm and humid. It feels like it’s missing, vibrations very noticeable. Wonder if it’s electrical. LMK if you have made any headway.
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Jonny Quest
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Joined: August-20-2013 Location: Utah--via Texas Status: Offline Points: 2978 |
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Posted: March-28-2023 at 2:56am |
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My "list" of possible suspects:
1. Accelerator pump (diaphragm in particular) 2. Accelerator pump linkage that is not properly adjusted 3. Nozzle / squirter size
JQ
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Current
2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
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Mille1sj
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Posted: March-27-2023 at 8:07am |
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Mine was not noticeably restricted, but I went with a larger size which seemed to give it the extra fuel it needed at the moment you put the throttle down.
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brhillman
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Posted: March-26-2023 at 8:43pm |
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Was the pump nozzle noticeably restricted? Mine looked fine last time I had it apart but I’ll definitely check it again when I get it back on the bench.
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Mille1sj
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Posted: March-26-2023 at 12:27pm |
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I had the same issue on our 77 Ski Tique after a rebuild. While at idle or low cruising speed, it would bog or hesitate going to WOT. I change the accelerator pump nozzle and the issue was fixed. Didn’t end up needing to change the cam on the carburetor. I ordered a few different size nozzles to try out.
Good luck! |
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brhillman
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Posted: January-25-2023 at 6:42pm |
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For sure. Didn’t solve the problem last summer, planning to pull the carb back apart before recommissioning this spring. Will definitely update this thread when/if it’s resolved.
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KevinGodwin
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Posted: January-25-2023 at 6:09pm |
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Wow - glad I stumbled on this thread! I have a '90 SN with the same problem. I have had the boat for 17 years and this problem only started this past summer and I have spent too much money at the marina trying to fix. Monkeying around with the accelerator pump did improve things but the damn thing isn't consistent. Some times its smooth then all of a sudden its rough. The secondary idea sounds interesting. Please let me know what the fix is when you find it!
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brhillman
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Posted: August-29-2022 at 5:30pm |
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A small update, I'm still chasing this issue. I had ordered the assortment of accelerator pump cams, thinking maybe I needed a bigger pump shot as the blue cam might give, thinking I had the white cam on there which is one of the smaller cams. Well, after pulling the cam out, while it was white in appearance it was in fact a 427 cam, same as the blue. So, that's probably not the issue, but might try swapping to a smaller pump cam anyway just to see if there's any change.
Some other thoughts though and an update on my thinking: I had the carb apart on the workbench again a couple weeks ago, and noticed one of the high speed air bleeds on the second side appeared a little varnished over. I cleaned these out pretty easily with the bristle from a brush and blew out with carb cleaner. I finally got a chance to test this again yesterday and...no change, BUT this did get me thinking about the secondary circuits and what else could be causing this bit of bog/hesitation on hard acceleration. To be clear, this hesitation I'm experiencing is not just a failure to immediately accelerate from idle, but rather it seems like the boat actually starts to accelerate for a split second, then falls flat, then picks back up. Does this sound like it could be an issue with the secondary circuit? I think Mark suggested earlier visually inspecting the carb operation under load to see when the secondaries open, with the thought that perhaps they could be opening way too early, which I think would create an extremely lean condition initially since the vacuum wouldn't be there yet to start pulling fuel through the secondary jet holes in the secondary metering plate. The engine would then pick back up as vacuum in the carb caught up to start pulling fuel down through the secondary venturi's. I haven't done this check yet, but it's next on my list for the next time I can get the boat out. I've also heard of folks testing this by temporarily wiring the secondary linkage closed. If the hesitation goes away, then that seems to point to an issue with secondaries opening. On thoughts as to what would cause this though, perhaps a too light or worn out spring in the secondary vacuum diaphragm assembly? I think a ruptured or poorly sealing diaphragm would just mean that the secondaries never open, which I don't think would cause this issue but rather a limiting of top end, is that right? One last thought, when I rebuilt the carburetor, the secondary sealing plate (the thin stamped plate that goes between the metering plate and main body to seal off the power valve cavity) was in pretty bent up shape, probably from a previous mechanic prying on the plate trying to pull it off an old gasket surface. I tried my best to bend it back into as flat a shape as possible, but now I'm wondering if there's maybe some kind of internal leakage here between the metering plate, sealing plate, and main body of the carburetor. Any other thoughts much appreciated, will update again once we get the boat back out on the water without our toddler running around so I can do the secondary test with the engine cover up.
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steve600
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Posted: August-16-2022 at 12:59pm |
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Great news! Definitely keep playing with the accelerator pump, you’ll get it
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brhillman
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Posted: August-16-2022 at 12:57pm |
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A small update: the big hesitation and backfire through the carb when revving fast in neutral seems to have gone away since replacing the ignition coil, which is good news. I am just left with a slight hesitation on acceleration under load. Tuning the idle mixture under load didn’t seem to have too much impact, but did notice when I tuned the mix screws that they didn’t have too much of an impact passed about a full turn or so out. I have an assortment of accelerator pump cams arriving today, will see if swapping the white to blue makes any difference.
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brhillman
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Posted: August-12-2022 at 9:12am |
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Yeah for sure, thanks for the suggestion!
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swilliams
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Posted: August-12-2022 at 6:57am |
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Mine was a Quadrajet so it entailed having the throttle plate re-bushed. I am not that familiar with Holleys but the same principle applies. You could check while engine is running the same way you checked for gasket leaks. Worth checking out as it could be that simple!?
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1976 Martinique,350 Omc. 1975 Glastron/Carlson CV16,115 Merc.
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brhillman
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Posted: August-12-2022 at 1:41am |
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That’s a good thought, I haven’t really checked that, but they are loose enough that if I fiddle with the accel pump too much with the engine off without opening the primaries and let fuel pool on the primary butterflies it will leak out a little through the throttle shaft. How did you tighten that up on yours when you had that problem?
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swilliams
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Posted: August-11-2022 at 11:27pm |
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Have you checked your throttle shafts for excessive play? I ran into the same situation and found that they were pulling in too much air, creating a stumble.
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1976 Martinique,350 Omc. 1975 Glastron/Carlson CV16,115 Merc.
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brhillman
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Posted: August-11-2022 at 8:00pm |
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Some updates and thoughts.
I discovered today that I *can* reproduce the hesitation (and even get a backfire) when running with no load on the hose if I try to rev the engine too quickly from idle. I never noticed this before, probably because I'd never tried to go from idle to WOT immediately when running without a load on the engine. I don't think this is expected behavior, but correct me if I'm wrong; I don't think I've ever gotten a backfire out of a car by revving it too fast. Armed with the knowledge that I seem to be able to reproduce the issue in the driveway and don't have to put the boat in the lake to test every time I make a change, and knowing that I still apparently have an issue and my previous efforts have failed, curiosity got the better of my and I tore back into the carburetor. I was also a little worried about the two little o-rings that go on the ends of the accelerator circuit transfer tube (probably not calling that by the correct name, it's the piece the goes between the metering block and the body of the carb and feeds the accelerator nozzle, Holley part # 26-23 I think it is) that looked a little torn. So, I pulled the carb back off, replaced the o-rings (had to buy a Holley rebuild kit to get them, my local Napa didn't have the o-rings themselves and we're taking the boat to the lake this weekend and don't have time to order, so those were $50 o-rings), put a tiny amount of lube on them so they wouldn't tear putting the metering block back on (the mistake I made last time I think), and replaced the power valve with a 6.5 while I was at it for good measure. Checked out the pump diaphragm and everything looked fine there, no tears or anything. It's also a new green one out of the rebuild kit I put in last fall. Put it back together and...still hesitates and wants to backfire on me if I open the throttle too quickly. I pulled the flame arrestor off and worked the throttle a bit (being very cautious to keep everything away from the top of the carb and to not work the throttle to point of backfire) and do see a nice spray of fuel out of the nozzles when working the throttle, right up till the primary venturis start spraying as well. Observing a carb in action is kind of a beautiful thing... Anyways, still seem to have a hesitation. Thinking about trying a different pump cam, but I don't really think that's going to solve the issue, and I'm starting to wonder if it's not carburetor related after all. So 1) I think I need to take timing measurements at various RPMs and make sure my timing is advancing in a reasonable way to make sure advance weights aren't sticking or something (even though I already checked this), and probably first verify my TDC mark is correct (thanks JQ for that suggestion) but 2) random long-shot, would an exhaust restriction present this way? I started to notice a strange occasional and irregular knocking sound, and today while running the boat on the hose I think I narrowed the source down to the muffler (my 89 has the invertaflow muffler). Kind of sounds like something is floating and bumping around in there, so I'm wondering if maybe some baffle came loose (I have no idea what these things look like inside) and is partially blocking the flow through the muffler. Thoughts? Sidebar: Good news is I'm getting better at swapping the carburetor on and off and pulling the bowls off, but I still hate that inverted flare fitting on the fuel inlet.
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brhillman
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Posted: August-11-2022 at 10:29am |
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Thanks Steve, that’s helpful to know.
Started on the hose this morning to check for fuel leaks after getting the carburetor back on last night. Wanted to check for vacuum leaks while I had it running, so sprayed water all around the carburetor base and could not notice any change in idle RPMs. Didn’t trust it so I repeated with carburetor cleaner and again could not detect any noticeable change in RPM, so I don’t think I’m looking at a vacuum leak there. |
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steve600
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Posted: August-11-2022 at 7:33am |
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Keep focused on the accelerator pump, sounds like you’ve done a thorough rebuild and lots of research. My experience in off idle hesitation on three different 4160’s has been solved with small adjustments to accelerator pump and cam while out on the water. I have really no technical facts of what helped each one, just learned on each that very small adjustments to the arm made a big difference, the cam in my experience didn’t really change alot. All power valves were 6.5. I think what helped alot was tuning the carbs under load. Not sure if this will help much but just my experience with hesitation on these carbs.
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brhillman
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Posted: August-11-2022 at 12:43am |
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Thanks for the vote of confidence Ken. Mark, that's a good idea, I'll definitely do that this weekend if it's still bogging after my adjustments.
After speaking with Johnny Quest today, I couldn't leave well enough alone and pulled the carb back off to check and blow out the accelerator pump circuit, and confirm the nozzle size and power valve that were installed in it. Also sought out the Holley listing chart (http://documents.holley.com/techlibrary_carb_numerical_listing.pdf), which shows that, based on my list number (50463), this carb should have originally come with a 0.025 nozzle, and a 125-25 (2.5 inHg) power valve. The nozzle on there now is indeed stamped 25, and I had installed the 5.0 power valve that came in my 703-29 rebuild kit, which I think should be a better fit for this application than the lower vacuum 2.5 power valve that Holley lists. Based on what I've read and heard, it sounds like a 6.5 or so would be even better, but maybe 5.0 is close enough for the moment.
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brhillman wrote: