Hesitation at low revs when warm

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davesmith84

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1972 1600 twin port engine, 34 pict 3 with a vac distributor but with pertronix electronic ignition module.

I've owned my bus for 4 years now and I've always had hesitation at low revs when warm. As you can imagine over the years I've done many services so it's extremely unlikely that it's anything simple like a serviceable item.

It runs amazingly, perfectly smooth whilst on choke. Obviously makes me think it must be running too lean when warm? But I've reset the carb when the engine is warm many times and the exhaust is black.

It seems to be at it's worst the hotter the engine is, this is what makes me think it's an intake air leak. I've tried spraying carb cleaner around the engine so many times I've lost count, no change in engine revs.

Other things I've tried (which have made no difference):

• Rebuilt the carb checking and cleaning all jets
• Recently re-bushed the throttle spindle on the carb
• Checked fuel cut off solenoid and is fine
• Tried the unscrewing of the idle/pilot jet slightly but this made it run worse. It's now just nipped up to the carb body.

The problem I have is the hesitation is so consistent and reliable, it kinda feels like it doesn't matter what I do, it will always be there. I've had it for 4 years now and counting! The bus is still very much drivable but only if you drive through the hesitation or let your foot off the pedal and then build the revs more slowly. Occasionally, when pulling away, the bus will hesitate so badly that if I didn't remove my foot from the pedal it would stall, or it feels like it will.

The only thing I can think is left to do is pull the engine, remove the intake manifold and re-seat all joints with new gaskets making sure everything is torqued down properly. It must be an air leak that just isn't showing when I spray carb cleaner around? The only issue I have with this idea is that if I cover the carb throat with my hand the engine dies instantly. I understand this normally means you have no leak? Maybe the air leak only 'opens up' when the engine is being run on the road and is hotter? Obviously the only time I can check for leaks is when the engine is idling which inherently means it's cooler than when running? Also, the hesitation obviously only happens on the road when the bus is under load. I can't make the engine hesitate by operating the throttle arm by hand.

Thanks for reading and I massively appreciate any help offered. :D
 
Try adjusting the accelerator pump, had the same thing on mine and that cured it. When you open the throttle it might not be getting a strong enough shot of fuel. Slacken bolt and turn it anti-clockwise, might be worth fitting an M6 nylock nut on the other side too. Mine kept coming loose and slipping.
 
Sorry, just have said that I've already tried that. I've tried it all the way + and all the way - and three places in between. Made no difference other than worse hesitation in certain places.
 
You have just explained word for word exactly the problem I have with my 71 1600 TP. I have checked everything valve clearance, timing, air leaks, compression. I have also done away with the points and replaced distributor cap, rotor arm, and coil but still the same!!
I have noticed that if I pull the ht leads off on cylinders 1 or 2 the engine revs reduce significantly whereas if I do the same on cylinders 3 and 4 the effect on the engine is almost insignificant?
I was convinced that it must be down to compression but all cylinders checked this morning giving a healthy 125 to 130 psi.

Sorry I don't want to hijack but I am very interested to hear the thoughts (as you are) of the knowledgeable folks on here. 8)
 
No worries Nelly, a problem shared is a problem halved!

I've also done everything you have other than a compression test.

Have you changed your inlet manifold boots? I'm thinking that's my next bet.
 
Yes, I did that a couple of day ago.

Have you checked your servo pipe as mine was leaking like sieve which when I noticed I thought I had cracked it....but alas not!
 
I am now thinking of buying a new carb but I still cannot work out why cylinders 1 and 2 appear normal but 3 and 4 are not and why does run so well when cold?
 
I'm just going to finish my dinner and then run the bus up to temp and try pulling ht leads to see if mine is behaving similar. :roll:
 
It will be interesting to know what happens.

Just checking but but you don't have a heart condition, you may get a little shock!! :lol:
 
davesmith84 said:
Sorry, just have said that I've already tried that. I've tried it all the way + and all the way - and three places in between. Made no difference other than worse hesitation in certain places.

Ah, must admit I was only skim reading the post earlier :lol: is the coil ok? They can play up once hot.
 
Or try a new king lead between the coil and dizzy.

Failing that, borrow a dizzy from someone with a known good engine and see if that helps.

Is the hesitation prolonged? Or just a stutter? Prolonged would suggest fuelling is a likely culprit.


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Speaking for myself, it is prolonged once the engine is hot and I also thought fuel so replaced the fuel filter but still the same ??
 
Not sure if prolonged. I can keep it prolonged or I can choose to drive through it. I know that sounds weird but I'm so familiar with how and when the hesitation occurs.

If I nail it constantly I never get a problem. Obviously I don't really like driving like that.

Right, I've just been out and no matter what HT lead I pull the engine revs drop.

I thought I would just check my timing. According to the Bentley manual I should set it to 5 ATDC which is what I think I have it set to. I have one V notch on my pulley which I thought was TDC. I have been setting the timing 5 degrees to the left of this mark.

I've just done a bit of googling and it seems that a dent is actually TDC. I've been out and found the dent but it's to the left of the V mark. This now means that my V mark is 7.5 degrees BTDC!?

I've just checked on http://www.oldvolkshome.com/ignition.htm and for my distributor it says I should be set at 5 degree ATDC but this would mean the v mark on my pulley is useless? Maybe my pulley isn't the original?

Anyway! I thought I'd set my timing to the V mark with a strobe like at 850 rpm and it's definitely got rid of most of the hesitation. I still have slight hesitation when I shift up very early but it's hardly noticeable.

What marks do you have on your pulley Nelly?
 
At the moment I am set at 5, but I can't believe that it is down to timing...but happy to be proved wrong??
 
I'd almost certainly say that's what your problem is!

Set it to 7.5 before tdc with vac lines disconnected and blocked.

5 degrees after tdc is only set with distributors that have dual vacuum.
 
I have played around with the timing before but I will certainly try setting 7.5 today and see what happens. Thanks
 

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