'91 Hard Starting When Warm

[Brynjminjones]

I'm sure this has been asked 100 times, but I'm struggling to find a real solution through searching.

My '91 GW, with 360 and factory 2150 carb, struggles to start when warm.

It starts perfectly cold, and starts perfectly within a couple of minutes of shutting it down when hot.
If you leave it longer (like going into a shop) it will always struggle. It takes a good several seconds of cranking, plus lightly opening the throttle to get it to fire up. It then runs rough for a minute until it goes back to normal.

This has always been the same regardless of ignition timing, carb settings, emissions equipment etc.

My carb is completely stock and has been gone through several times. It is sat on top of the factory spacer.


There are a few clues I've noticed which have led me to suspect it's boiling the fuel out of the bowl.
If I leave it with the hood up and air cleaner off, I don't have this problem at all. It will always start on the first flick of the key.

Sometimes I'm sure I can hear the fuel boiling in the carb.
Also, the other day I went out for a drive then parked at home. 10 minutes later I had my head underneath the rear axle, and I could hear what sounded like fuel returning to the gas tank.

I'm guessing this is the "vapor lock" I always hear about, but I'm not sure what to do about it when I already have the carb spacer installed.

Can anyone advise on whether I'm on the right track, and how to improve it?

[MysticRob]

Fuel filter clocked correctly?

[Brynjminjones]

I forgot to mention that - yes it is, first thing I checked!

[Harrinad]

A couple things to think about, when I had problems with my 360 I did a diesel swap because I'm dumb, don't do that. On the up side, I've never had carb issues since...

I think the 360 has a vacuum choke which could make it difficult to start warm if it's not disengaging correctly. It also has a vacuum advance which could be an issue.

When you say your carb is boiling fuel it throws a red flag for me, it should never be that hot. Do you think it's possible that you have a couple burnt valves that are letting gas get back through your intake? Do you ever have any backfiring?

You may do a once over on your carb, lots of things can make it tough to start like partially clogged jets, worn needles, stuck or misadjusted float, etc.

[Brynjminjones]

Thank you, I'll do my best to answer those points...

The choke is confirmed to be working correctly. I spent ages last year getting it dialled in! It opens with both heat and electrically, and the pull off works great.

The vacuum advance is also good.


The carb has been taken off and cleaned several times. Nothing at all seems to be wrong with it and it's very clean. The float etc all moves just fine. I went through all of this whilst chasing other problems previously!

I think the fuel boiling really must be the cause.
I don't ever have backfiring, the engine was actually rebuilt about 6,000 miles ago and runs beautifully.

One thing I looked at yesterday was the fuel filter again. I noticed that it's the right way up, but was positioned a little low, resting on top of the valve cover (possibly transferring heat?). This meant that the return line was at the same level as the carb inlet.

I've adjusted it now so the return is higher than the carb inlet, and the filter is no longer resting on the valve cover.

I should mention that this isn't a new problem at all, it has always done this consistently regardless of everything else.

I will drive it today and report back on whether the filter position has made a difference!

[wiley-moeracing]

Did you ever put a thick spacer under the carb( 1-2 inch) to keep the heat from the manifold transferring to the carb?

[babywag]

After it heat soaks for a while...
Pull air cleaner, open throttle, watch for accel pump shot.
Is it good/strong spray?
While throttle is open look @bottom of intake floor, is there a puddle of fuel sitting down there?

2 things may happen...
The heat makes the fuel in bowl evaporates quickly leaving you with little to start engine. e10 here in CA is problematic imho. Nothing like real gas without ethanol blend that doesn't do it.
Or fuel dribbles out/emptying bowl and puddles in intake leaving an uber rich condition on restart.

[FSJunkie]

What you describe is 100% normal.

A fully warmed up engine will start immediately if it is started within a minute or so of being shut off.

Longer than that allows the lack of air movement around the shut off engine to cook the fuel in the carburetor. The fuel boils and forms a thick vapor fog that fills the carburetor throats and intake manifold. This fog creates an over-rich condition that the engine will not start on until it is cleared out. It's basically a slight flooded condition.

Operating the starter with the throttle held partially open will clear out the excess fuel in a few seconds, five seconds at the most, and it will start. It may blow some visible smoke out the exhaust and run rough for a few seconds after starting, possibly requiring you to hold the engine at a fast idle speed until it completely clears out the excess fuel. Then, it'll be ready to go.

Every single carbureted engine I've ever owned (roughly a dozen) and nearly every one I've ever driven (hundreds) does this.

Get in.
Open throttle 1/4 to 1/2 way.
Operate starter until the engine starts in under 5 seconds.
Hold throttle at fast idle (1500 RPM or so) until it runs smooth, then let off to regular idle.
Go into gear.
Drive.
Think nothing of it.

[DarkMonohue]

How fresh is that fuel pump? Mine was getting tired (i.e. probably had a slightly leaky diaphragm) and refused to fill the carb after it sat for a week or two. Yours may be a variation on that theme.

Otherwise, mine seems incredibly sensitive to throttle position on warm or hot starts. Sometimes it lights off immediately and sometimes it just wants to embarrass me. I've simply accepted the notion that cold starts are pretty predictable, and warm or hot starts aren't.

[Brynjminjones]

Thank you guys. Sorry, not had much time to look at this over the weekend - I hope you all had a good Easter!

My carb does have the factory spacer (plastic thing with a very thick gasket on top) but nothing else.
The fuel pump was new last year too, and didn't have any effect on this symptom!

Tony, I've not had the chance to investigate that yet, but I do know that if I pump the gas once before starting in this situation, it takes even longer to start.
I think it might be evaporating, going into the air intake and causing a rich condition on startup, like you and FSJunkie said


One question - the carb vent solenoid is stuck closed, so it only has the two vents on top of the carb.
Could that be part of the problem? I'm guessing that is causing all the evaporated gas to go to the intake, rather than the charcoal canister.

[DarkMonohue]

One question - the carb vent solenoid is stuck closed, so it only has the two vents on top of the carb.
Could that be part of the problem?

Absolutely. This is definitely a case where you want to fix the problems you know you have before looking elsewhere.

[Brynjminjones]

Absolutely. This is definitely a case where you want to fix the problems you know you have before looking elsewhere.

I usually think that way, but I've read so many people claiming that blocking off the vent makes no difference!

I've investigated further, as I thought the carb vent solenoid was stuck closed. Looking further into it, it turns out the solenoid works just fine, but the idiot PO has shoved a vacuum cap inside the vent tube on the carb, then put the hose back on anyway.
Unfortunately I managed to snap the plastic on the solenoid whilst investigating, so I've epoxied it back together in the hope it will be strong enough.

I will put it back together and take it for a drive as soon as I can, to see if that was my problem.

I really hope so, as I've been putting up with this for ages!

[MysticRob]

Might be a big ask, but could you take a couple pics of that solenoid and associated plumbing around the carb and PM me? I'm missing those particular things since a PO removed them, and I'd like to make the system whole again before I do a fuel injection swap at some point. I like my resale/originality so any help there would be appreciated!

Rob

[babywag]

What is your normal fuel blend over there?
Any chance of trying a tank of real gas w/o ethanol?

If stock air cleaner still being used? is the inner flap operating correctly?
If it were to be stuck closed/not opening that would exaggerate any rich condition a bunch. Inner flap should always be open whenever the engine is running.

[Brynjminjones]

Rob, sure thing. I'll get some photos and send them over. Give me a prod if I seem like I've forgotten!


Tony, I think we're 5% over here, which probably doesn't help. I would be interested in trying better fuel, but it's already expensive enough over here!

I have the stock air cleaner from a '79, which doesn't have the inner flap at all. The outer flap is stuck in the cold air position.