compression and idle quality

Got a situmation. No idle from 1000 RPM or less. Stalls in neutral and will immediately stall when going into gear. Can coax into gear go full throttle and runs like a scalded dog. Back to idle and stalls. Dont matter where the mixture is or idle rpm. And I get a good responce from idle circuits and have no flooding issues. It will idle and start to break up and die in neutral and will stall when going into gear. Got a vacuum gage on it. And at idle will watch vacume fade and engine will stall. Hotter she gets the worce it gets. I can crank it and throttle it and will idle some times but will break up and vacuum will drop and stalls. Im wondering if I got a ring problem affecting vacume at idle. Kinda reminds me of a two stroke with fried rings. Runs good till warm then goes to ***************.

Ive determined its not in the carb. As I have had three different ones on it. All act the same. Checked the valve at the tank. Its good. Fuel line clear from valve at tank to fuel pump. Fuel pump was suspect but how would it run a motor at wide open throttle and not at idle? Suspected vaporizing fuel in line from pump to carb. Removed t stat and engine temps dropped to 110* from normal of 160* still same issue. Im thinking low compression affecting idle qaulity by way of weak vacuum signal to carb causing stall. But yet question that theory because there is absolutely no exhaust smoke at all.

Sounds like a vacuum leak develops once the engine warms up. Intake gasket maybe?

Vacuum gauge is hooked to the carb or carb spacer, I presume?

Could be a valve issue. Just had that problem with my '93 Honda ended up being a bad valve.
Do a compression check then if need be a leak down.

I will be doing a compression check. That was next on my list. Went t ok the tool box to find it missing. I must have loaned it out and the loanee forgot where the hel it come from. :-/.

You might consider doing a leak down test. It is more comprehensive than a compression test; can help you isolate valve vs. ring issues.

Vic, Pull the PCV and take a piece of paper and hold it over the PCV hole in the valve cover. This is a way to check for a vacuum leak from the bottom of the intake into the engine. If it pulls it it's pulling vacuum.

phatsat67 wrote: Vic, Pull the PCV and take a piece of paper and hold it over the PCV hole in the valve cover. This is a way to check for a vacuum leak from the bottom of the intake into the engine. If it pulls it it's pulling vacuum.

Well that was also looked at. Actually accidentally. Pulled the pvc and blocked the line trying to get it to idle. While the pvc was out noticeable blowby was evident. Its not choo choo puffing but a steady flow without any positive pressure behind it. Also once I pull the intake oil in the intake ports of the head would be tell tell sign of vacuum leak under the intake..

I agree with TR Benj, Compression does not come and go hot and cold like your problem. With no compression they get hard to start in addition to other problems, smoking exhaust etc.
It very much sounds like a heat related vacuum leak, the block and heads expand as the engine warms up and a crack that is sealed cold can open hot causing issues. As a twist, have you overheated this engine and maybe you are running with partially collapsed exhaust tubes that are shutting down your engine at idle but open under full throttle?

A compression test will tell you the overall health of the engine, while the plugs are out read them and see what you learn from reading the plugs. Champion, NGK or Autolite all have websites with great pictures on reading plugs.
If you have a fuel supply issue the plugs will be very clean from lots of air and not much fuel. I would read the plugs before working on the fuel system. Let us know what you learn. I hope it is an easy fix.

Ok heres the cold compression numbers
#1/162 #2/166 #3/170 #4/170 #5/163 #6/165 #7/165 #8/168.

Dead headed fuel pump with guage will rolling compression test. Five pounds max with fuel pressure gradually dropping between cylinder tests. Plugs are ghost white with a faint chocolate color covering maybe 5% of the porcelain insulator.

Going to run it and do a hot compression test now and get those results then temp install electric pump and see if it will repeat symptoms. Then I guess ill move over to ignition and electrical. Wonder if a weak battery would cause all of this as I also have noticed that muiltiple starts will weaken it to the point of no roll over pretty fast.

What ignition are you running?

hotboat wrote: What ignition are you running?

MSD HEI with MSD Module cap rotor wires and delco plugs.

Definitely could be the alt. Maybe it's charging at first and then stops? Getting a little charge to the battery and when it stops, the battery isn't enough to keep the MSD running. MSD's don't like any kind of power hiccups. They can be finicky that way. You said you have a MSD HEI and ign. module. I assume that means the whole MSD dizzy and ign. box setup?

If so it's possible that the MSD is on it's way out too, although unlikely. It either works or it doesn't. Don't rule out the mag pickup though. I'm guessing if you put the MSD in, you're aware that you can test the ign. box real easy. That is if you didn't hardwire all the connections so that you can disconnect them if need be. If you hardwired them, it'll be much more difficult to test.
First unlpug the coil wire from the dizzy cap and set it up so the contact lead is a half inch from ground. Unplug the dizzy wire going to the ign. box, turn the ign. key ON and then short out the two wires going to the ign. box (the ones you just unplugged from the dizzy). When you remove the short, you should get a bright white spark from the coil wire to ground. You'll hear that sucker too. It really snaps pretty loud. You're basically simulating the short/open from the mag pickup. If that checks out, the ign. box and all your wiring is good. You should get that bright spark every time you short/open that connection. The only thing left is the mag pickup at that point.

I had to replace mine several years ago. Was giving all kinds of fits. Work and not work. Real intermittent. I keep thinking replacement was $20-$25 or something like that.

Thanks backfoot. I have the dist. Not the muiltispark box. I was thinking of addding one though.