Measuring vacuum, rough idle.

Morning Rob! You, like many, are searching for that elusive key start and smooth idle from an older carbed boat.

The first red flag we see is that the idle circuits don't have much effect on how the boat runs. They generally should kill the boat if only one is turned all the way in. This could mean a few things.

-Sometimes folks have the timing retarded too much at idle which causes the throttle place position to be open so far that the engine isn't actually running on the idle fuel circuits. Your timing seems in the normal realm. Just make sure you are using a quality adjustable like to verify proper timing.

-As for your vacuum question, Marine engines have less places to hook into vacuum than a car does. You need to get some type of adapter to fit the gauge to the PCV nipple on the carb spacer on the rear. Idle vacuum should be set at desired in gear idle rpm. You should see a number higher than 15 but more desirable around 16-17 inches. If you can't achieve a number similar to that the thing probably has a vacuum leak somewhere.

An issue with some higher hour carburetors could be a vacuum leak coming from worn throttle shafts/base plates. I had a carb with this issue that made me scratch my head quite a while.

So a vacuum leak can also cause the issue you are seeing. The symptom of vacuum leaks is usually an ok idle out of gear but a stumble into gear and hard to operate properly in gear at idle. It also makes the idle screws not very responsive.

If the carb isn't fresh you might as well go through it to verify and check the power valve. Holleys aren't rocket science during a rebuild.

I would start with the Vacuum adjustment and report back.

A little more background on whats new/rebuild/replaced on the engine would be helpful.

Had same problem went to take it apart and found out that my bowls had come loose tightened it up and solved problem. Not to say that's what you are experiencing, but it helps to check all the simple stuff and eliminate the easy stuff first.
mike
Be careful the bowls won't take having to much torque.

phatsat67 wrote: . An issue with some higher hour carburetors could be a vacuum leak coming from worn throttle shafts/base plates. I had a carb with this issue that made me scratch my head quite a while. So a vacuum leak can also cause the issue you are seeing. The symptom of vacuum leaks is usually an ok idle out of gear but a stumble into gear and hard to operate properly in gear at idle. It also makes the idle screws not very responsive. .

I have been scratching my head all week trying to figure out why my boat occasionally runs very rich at idle and sometimes the idle screws snub it when I bottom them and sometimes they don't. Sometimes the I get flat 16 inches of manifold vacuum and sometimes it bounces between 5-10 inches.    Finally figured out tonight that there is a vacuum leak at the throttle shaft. I can snub the motor with carb cleaner from the outside.   Float is set right, no seepage from the boosters. Is it agreed that a throttle shaft leak can cause the idle circuit to not operate correctly? Any options for repairs outside of replacement carb? Have seen mixed reviews on other forums of drilling out the bore and putting in a bushings.

They do have decent tech help, I've used them in the past. But nobody is home right now when i tried to call. I have seen in other posts that people say that the throttle shafts in marine carbs are different as a safety measure to prevent fuel leaking. Is this true?

I wouldn't mind paying $140 to just get a new throttle body without fooling around with reaming the bore in putting in bushing.

wiscofoot wrote: They do have decent tech help, I've used them in the past. But nobody is home right now when i tried to call. I have seen in other posts that people say that the throttle shafts in marine carbs are different as a safety measure to prevent fuel leaking. Is this true? I wouldn't mind paying $140 to just get a new throttle body without fooling around with reaming the bore in putting in bushing.

Wisco

The marine shafts are different

Click on the link for an old thread with some info and pictures   

link

Didn't think you would that's why I advised you to call. They might not even be there- might not be a essential business Your other alternative is to spend the money and look at a quickfuel. Took me awhile to find a 450. I had seen them earlier that year and thought well they are available I'll just wait.Then when I was ready none to be found. I wasn't having trouble I was going to convert a 2 bbl to 4 and was going to use the old 4bbl off the Mustang and put the new one on it so I could afford to wait.

wiscofoot wrote: Thanks Ken. That thread was hard to take. I am happy to see that some of the participants haven't been online in almost a decade.   I will return my throttle plate when it arrives, instead of trying to plug the vacuum ports.   I have been in two boat fires, neither of which were caused by non-marine parts, but I will never put myself or others at risk of having to go through another.   Does the fact that the shaft are splined impact the efficacy of reaming and bushing the shaft bore?   FYI I didn't find anything good for a reman on eBay as of May 1 2020

The bore that the grooved shaft rides in is smooth, so the old shaft should be good with the new bushings.

If the issue is leakage at the shaft like you think, then you should be able to wiggle the shaft if the wear is bad enough to cause running issues.

I'd figure that you have other issues with extra fuel getting into the engine from someplace besides the primary idle circuit.

Do your secondaries shut all the way to the stop screw every time or maybe hang open some and your idle issues are because of that?

Post a picture showing the left side of the carburetor showing the primary and secondary linkages and the little sorta S shaped linkage that connects them.

Take the picture with the engine off and the throttle in neutral.

Thankfully my better half is working from home so i was able to solicit some pictures on demand.

I am going to pull the carb off tonight, a guy i work with happens to have a reamer and bushings so if the shaft is noticeably loose i can at least make that repair at no cost. Could an obstruction in the air bleed cause this?

I will post pictures of the bottom side once i take it off. If anyone has leads on a good reman unit please let me know.

next one need to put some finger pressure on the secondary's bellcrank (onthe right) to see if stays firm, or can rotate a bit , leaving a gap between the where it is now, and the bent rod.. If an 1/8" or more shows, the primary idle stop is cranked cranked up

That linkage needs to be like in the picture I posted.

Any gap like what GottaSki mentioned means the secondary butterfly's can flop around a little.

There's always fuel coming from the secondaries at idle.through the secondary side idle circuit All fixed orifice sizes in a 4160 like yours, nothing to adjust

As far as the Ebay carburetor................. run away run away as fast as you can. We'll just leave it at that

wiscofoot wrote: I just realized that I hijacked this, so I apologize to Robbied because I Robbied his thread. Before I pulled the carb off I decided to check the power valve and it was blown out. Luckily I had a 7.5 and gasket leftover in the Holley kit and it completely resolved the issue. I was able to tune the idle adjustment screws for peak vacuum and have excellent response, steady smooth idle, and no smoke.   Thanks for your help guys. Not scrapping the turd yet! May still ream and bush the shaft bore but for now I will bask in the pleasure of resolution.

It seems like you should have checked or replaced that early on in this troubleshooting process   

I guess since it was leftover in the kit, you decided not to replace it, maybe because it was a 7.5

I wouldn't touch the bushing job, you'll probably create more problems messing with the shaft and butterflies.

wiscofoot wrote: Ken, I agree that I should've checked it earlier. I didn't realize it was a possible source for fuel to dump in under the throttle plates. I feel dumb now. What do most guys run for PV in these? The marine kit only came with a 7.5 and a 2.5.

Yup, if the diaphragm is ruptured , cracked etc gas from the bowl gets sucked through it and into the engine through the vacuum port that connects to the power valve bypassing all the carburetor circuits.

If the diaphragm is good and the valve seat is leaking, then gas gets in through the normal flowpath for the power valve through the PVCO's and into the main well.