V8 hesitating when throttle first applied.

WarrenL

WarrenL

Active Member
We've mentioned this in my Members' Projects thread, but the problem should probably be thrashed out here in the relevant section.

Just tuned Brown Rover has developed a hesitation when the throttle is first applied. I push the loud pedal, the engine suffers a moment or two of indecision, then rockets away. Carbs have just been balanced and set, and the problem has made itself apparent since then. I believe it might have been evident in embryonic form right after tune-up but nobody believed me at the time.

Ron, I changed the fuel filter like you suggested and it hasn't made any difference.

Next, while the car was idling, I pulled all the plug leads off one by one. Interesting results. Removing leads 2, 3, 5 and 8 made no difference to the sound of the idling engine. Removing leads 1, 4, 6 and 7 made the engine stutter. What's going on here? Have I invented cylinder deactivation for the Rover V8?
 
Hi Warren,

My next port of call given that replacing the fuel filter failed to rectify the situation would be to investigate the fuel pump itself. Does Brown Rover have a mechanical fuel pump fitted and when was that last overhauled?

I know my Rover some years ago had a slight hesitation as you describe but in my case changing the filter cured the problem.

I have never pulled all the leads off that are associated with each carburettor, but I would have thought that it should make the engine run like a hairy goat. Are you able to record a video or a wave file on how your engine sounds with all and then half the leads fitted?

Ron.
 
Carbs have just been balanced and set, and the problem has made itself apparent since then.
Click to expand...

How confident are you that they've been balanced, and not 'balanced'!

I've had it before where you balance the carbs, it all behaves well. let the engine cool overnight and it's out again. A couple of iterations and got it right....

Rich
 
Definitely balanced, Rich. John tweaked the car over several days. Those cylinders: 2, 3, 5 & 8. Isn't that the pattern fed by one carburettor? The end two on one side and the middle two on the other? Could I have a vacuum leak or something affecting one carb?

Fuel pump is mechanical, Ron. Not overhauled in my time. But there's plenty of gas flowing through. I checked that out too, while the filter was off. Also, once my foot is down and the revs have climbed through the hesitation, the old girl pulls like a sex-crazed schoolboy. Correct me if I'm wrong, but I've have thought a fuel pump problem would have starved the engine at higher loads.

John says he'll take a look for me this Saturday, but I'm hoping to find something simple to save him the hassle, hence the thread.
 
Looking at the cylinder numbers that are not playing ball, it seems possible that one of the carbs is responsible. Each carb, i believe feeds the opposite two outer, and the same side two inner cylinders (or the other way round) Not 100% sure. Either way i suspect that there may be a carb issue (stuck float etc).
I'm sure someone will correct me if i am not 100% correct.
Hope this helps
Glen.
 
WarrenL said:
Next, while the car was idling, I pulled all the plug leads off one by one. Interesting results. Removing leads 2, 3, 5 and 8 made no difference to the sound of the idling engine. Removing leads 1, 4, 6 and 7 made the engine stutter. What's going on here?
Click to expand...

You have a problem with the O/S (RH) carb.
 
Surely got to suspect a carb problem if the symptoms came about after a carb tune up? As already said, if the symptom is brief hesitation on acceleration, this does sound like the carb that feeds cyl's 2 3 5 8 is not delivering the correct mixture (at least) on initial acceleration and nothing to do with the fuel pump. If pulling the HT leads at idle makes no difference, sounds like the carb is starving these cylinders completely at low rev's.

When the carb's were balanced, was the mixture on the carb's adjusted as well as just idle speed? I am wondering if the the mixture has been set far too week at idle on the RH carb? In other words, the jet height is far too high and the needle is blocking the jet in the idle position. Only when you give the accelerator some welly, will the carb piston rise and allow fuel into the carb throat (after a delay and, hence, the "hesitation"). Assuming your carb's are the same as my SU HIF6's, you could maybe try turning the mixture screw say 1 full turn clockwise to richen the mixture on the suspect carb with engine at idle. See if this brings cyl's 2 3 5 8 back to life? If not, just turn the screw back to exactly where it was?
 
That all sounds very plausible, JVY. I'll give it a go at the soonest opportunity (maybe tonight, although I think the wife has Plans) and report back. Otherwise I'll pass the info on to John when he takes a look on Saturday. I'm wondering if the float is stuck or something, because John spent a lot of time getting the car right and it seemed OK when I drove it home. In fact, it was great on the way home! There was some actual real horsepower, and no clouds of black smoke pouring out the back like a coal train. The hestitation has become apparent since.

Just another thought; when establishing that 2, 3, 5 & 8 were under suspicion, I pulled the plugs from 1 and 2 after the car had warmed up and was idling in the garage. 1 was slightly black, and 2 was almost completely clean, like it had been burning nothing. Didn't look at the other plugs, but I wonder if I'd find much the same?

EDIT: Just rang John and he's thinking along the lines of a leak somewhere. He's told me to selectively spray some engine start (happily, I have a can in the garage) around the carburettor and see if the revs rise at any point. Clever thinking.
 
Yes, could be a leak on the inlet manifold. I've seen similar symptoms on other cars. You don't need much of a leak to draw nothing but air into the engine at idle but when you get the rev's up, the decreasing manifold pressure is enough to pull the air/fuel from the carb into the cylinders?
Please go easy on the easy start - don't want any engine bay fires :shock: . IIRC, some other members on our forum have had unpleasant experiences with easy start (i.e. a case of external combustion rather than internal combustion :oops:).
 
as said watch out with the engine start.

Is it number 2 that has the servo vacuum take off in it? My servo pipe has split (albeit a very small split) and this (along with some other issues) made it a nightmare to start and run. Other issues sorted now, and I'm waiting for some servo pipe, but my money'd be on a vacuum leak.
 
Warning heeded. I'll take great care and keep the fire extinguisher nearby. The technique involves just careful small squirts around the places where a leak might be possible. Now, the inlet manifold has a nearly new gasket that has been retorqued a couple of times to make sure it is nice and tight, and that pattern of misbehaving cylinders would be very odd for a manifold leak, wouldn't it? I'm going with the carburettor theory for now, easy things first. Unless, of course, you're referring to the gasket between the carburettor and the manifold. The carbs were removed and replaced recently.
 
I've been out to the garage this morning and done a little fiddling. First of all I checked that the carburettors were securely tightened down, and retorqued the inlet manifold bolts. I started the engine, warmed it up and then carefully sprayed a little engine start about. Nothing. I checked the theory by pulling a couple of hoses and spraying gently near them, which caused a slight lift in revs. I put them back on and listened to my V8, which sounds not so much like a V8 as a slightly wheezy four cylinder. I tried the lift pins. Right hand side carburettor showed no response, the left hand side produced a slight lift in revs then a gentle settling.

I then removed the dash pot piston from the top of the RHS carburettor and put it back in. There was no perceptible response from the idling engine. I removed the left hand piston and as I pushed it back into place the engine stuttered and nearly stalled.

It seems to me, a layman of course, that the RHS carburettor has stopped working at low speed. Something has happened since it came home because the first couple of drives (including a decent blat in the countryside) the car was singing all the way. Am I on the right track?

Also noticed that the RH front cylinder is marked "2" on the manifold, so my previous description of suspicious cylinders might be suspect. Taking Number 1 as the LH front cylinder, I numbered them like this for the purpose of describing which ones seemed to be affected:

(front)
1 5
2 6
3 7
4 8 (closest to steering box)
(rear)

I guess it's actually:
1 2
3 4
5 6
7 8
 
SOLVED!

I got to the bottom of the problem today: the bottom plate on the RHS carburettor was leaking fuel in copious quantities. God alone knows how I didn't notice it during my previous investigations. My theory is (and please correct me if I'm wrong, because I'm a babe in the woods when it comes to carburettors) that the float chamber was draining at low throttle or idle, but the rate of fill was exceeding the rate of leakage with more prod, hence the problem easing when I had my right foot down.

Anyhow, gasket gunk has temporarily solved the problem, but the o-ring is pretty hard and starting to crack, so I shall source appropriate replacements for both left and right sides. The main thing is, my car is driveable once more!
 
So, now we know you have HIF carbs! And harvey thinks they're better than HS's, Grumble... (Sorry H, Couldn't resist!). On the basis that you have found one brittle O ring, there's quite a few more in an HIF. I'd buy the full monty overhaul kits so that you have the remainder ready for when you've enjoyed a little recreation with your newly road legal motor and are ready to tackle something else...

Well done on spotting and solving the cause!

Chris
 
Thank you Chris! Looking on the bright side, there's nowt like a problem that you desperately want to solve to make you learn new tricks. Until recently, carburettors were just an appendage that mysteriously mixed fuel and air for use in the business part of the engine, but I'm now starting to get a grip on how they perform their magic.

I will be ordering the kits, by the way. I don't intend to do anything for now because I'd like to just drive the friggin' car for once, but I'll have 'em on hand at the next sign of trouble.
 
The drama continued! Not long after returning to full operation, the car started playing up again. Except it was worse, and the carburettor seemed to stop working altogether. I took off the air filter with the engine idling (on four cylinders), sprayed a little engine start into the throat of the offending SU, and there was a brief firing of its related four cylinders. All Forum clever clogs who have been following this saga will probably come to the same conclusion I did at that point.

Yes, upon stripping the carb I of course found a blob of sealant blocking the main jet - so much for my bush mechanic temporary repair! By now I had the correct O-ring on standby, so I cleaned out the carb, removed my temporary sealant, installed the new O-ring, and Presto! we're up and running again.

Sometimes shortcuts, however temporary, just aren't worth the hassle.
 
I know Harvey's probably had to take Valium, but it was only a little puff for diagnostic purposes, not emptying the whole can into a non-starting engine in a futile effort to make it run. :)
 
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