Bogging down-help!

[listernoble]

I have a 1969 Ginetta G17 single seater which I hillclimb.
Th eengine is based on the Imp,bored and stroked to 1098cc.
Power 131.5bhp at 7988 rpm and torque 91ft/lbs at7255 rpm.
Cam .410 lift, 50-90-90-50
Twin Weber 40's 32mm choke,130 main ,F16 emulsion and 4.5
venturi.
I have had the car on the rollers twice to try and cure a severe stumble or bog down around 5000 rpm,the engine pulls as sweet as a nut before and after that.
The stumble occured at the same place ,like a mirror image before and after re-jetting carbs which increased torque from 77.4 ft/lbs at 7111 rpm and 108 at 7601 rpm
The bog down causes some real problems on tight corner tracks such as Wiscombe and Prescott.
The chap at the rolling road is good and says it is the cam,some of my pals have run the same cam in similar engines without this problem.
We tried altering the spring rates on the auto advance but I know I am running a lot of advance.
Ignition is old style Lumenition optronic and I can't use modern programmable electronic stuff as sometimes I run in Historic classes.
If it's the cam why is it OK before and after the stumble?
Question is,will other carb settings cure this, or is it likely to be ignition timing?
Lister

[StephenRae]

Sounds like a fuel supply problem to me.

[KC]

You probably have a fuel starvation issue happening at that particular rpm range. Its not uncommon to have an conflict between ignition timing and cam timing and they occur for you at this range and not at other speeds. You say you are running a lot of timing advance, how much is a lot? 30 degrees, 35 degrees? You can try to reduce your ignition timing, resulting in a loss of some power or maybe change the speed of the advance. We run an MSD Billet Distributor on an 8.8 liter (540 cid) Chevy V8 and adjust the advance counterweights so that they advance to max at about 2000 rpm. These are adjustable to give timing advance control. You are probably running something similar. Try changing the rate of change and see if you can get the issue to go away or at least move to another part of the rpm band.

[Tim Falce]

Have you tried to find out what jets and chokes your mates are running and try the same. webers have so many variables to alter it could be one of half a dozen things such as the accelerator pump jet, air bleed or the aux chokes.
I had similar problems with 45s on a differnet type of engine and it ended up being the chokes and aux venturi sizes incompatible.
Not much help am I..............

[listernoble]

Thanks for all the advice so far.
My own hunch is that it's a combination of aux venturi and ignition advance,fueling problem possibly as distributor goes onto full advance.
How much effect does the venturi have if it's oversize,which according to the tech literature it is?
Lister

[KC]

If the venturi is overly large, you may not have enough velocity to the intake to get enough air into the engine. An engine is nothing but an air pump, but at high rpms it reaches a certain volumetric efficiency and performance can degrade beyond that. A smaller intake can increase the air pressure at the venturi and speed up the airflow allowing more air to pass by the intake valve during the fraction of a second that it is open. That is the concept behind dual intake runner intake systems. A short tube allows high flow at low rpm and the longer tube has better flow at high rpms.

[listernoble]

But why will it run OK at lower revs and then hiccup around 5000rpm and then pick up again?
Admittedly it's not pushing out a lot of power below 5000 but it is running smoothly.
There is a distinct, short and sharp power drop between 4500 and 5500 rpm.
Lister

[KC]

The venturi may only be part of the problem. It would seem logical that above 5000 the problem would persist and not go away. On that I am not sure, but I would hazard that you are getting a cumulative problem. I know guys who have done exhaust work on their road bikes and start having problems with torque spikes and extreme drops in horsepower. Its difficult to get a compromise setup on any race engine and even more so on small displacement ones. There is only so much torque and horsepower you are working with and all of those things can upset the balance one way or the other.

[Red Dog]

Hmmmm. I doubt it is fuel starvation, more likely the reverse. If you have sufficient fuel pressure at the higher end of the rev range then you can be fairly sure you have enough throughout.

Is your ignition not fully advanced at 4000 rpm? Most race engines I know of are fully advanced well before 5000. It is not uncommon to find the advance fixed at the max when you have an engine with a power band from 5-8000. Retarding the ignition will help a bit if you are needing to use the power lower down. You would probably need between 32 and 35 degrees total advance on an engine like this. Forget fiddling with the springs.

You have a fairly long period cam anyway (320 degrees). I expect it comes on the cam at about 4500. You don't want it sweet as a nut low down, you want power from 4500 up!!

How has it been timed? A lot of people time on one lobe, or worse, on the opening and closing points which are really vague!! Try retiming the cam. What does the cam grinder recommend? It may need 3 or 4 degrees of advance from your listed timing. Cams can also vary a bit from the stated profile so you need to use a method of timing that gives you an averaged setting.

Easiest is to find full lift on inlet and then exhaust by going a few thou lift either side on each and splitting the difference in the middle. With the two accurate full lift settings you have the lobe centreline angle. The cam grinder should tell you what this angle is, but you need to check it to set the timing. Say it is 110 degrees then if you set inlet full lift at 110 degrees after tdc (exhaust should then be at 110 before tdc) the cam will be 'straight up' with no advance or retard. If you advance the cam by 3 degrees then full lift on inlet will occur at 107 degrees after top (exhaust correspondingly at 113 degrees before top) I am only guessing at your LCA because the lobes are not necessarily symmetrical.

The snag with retiming the cam is that you will need to revisit the rolling road each time you alter it.

You may have too much accelerator pump delivery. Try reducing the jet sizes or increasing the bleed-back jet sizes. I think that if you have too little fuel then you will hear the engine spit or stutter rather than bog. What air correctors are you running?

The other, perhaps controversial suggestion I have is tha maybe you are running too wild a cam duration. If you need a bigger spread of power starting lower down than you might consider reducing to 310 or 300 degrees duration, but with a bit more lift. What diameter are your inlets?

BTW that sounds like a very good power output at 131 hp.

The sequence for setting the engine up once you've checked the cam timing is to get the air fuel ratio right by playing with the jets, perhaps trying different emulsion tubes along the way, then when the mixture looks right on the CO meter, someone gets to stick their hands under the bonnet to swing the distributor for max grunt at a couple of different revs, say 5000 and 7000. Hopefully the same amount of advance will be required or you have a problem.

[listernoble]

Thanks for aall you r thoughts,I will chexk out and post answers.
i would like to post the dyno curve print outs but don#'t know how so may scan them in and e mail to you as the power drop is quite dramatic.
I have asked my mates to c heck out their carb settings,trouble is most of them have gone on to other Ginetta models.
Lister

[zefarelly]

Did you have your cylinder head tested on a flow bench ? its not uncommon for the gas flow to have glitches in it as valve lift and gas flow increases ? . . .this could then be attributing to the drop off at 5k ? I'd have thought a fuel or ignition problem would have been sorted on the rolling road, the exhaust probe should tell you what the mixtures doing ??? have you e set and checked the fuel pressure ?

[listernoble]

The rolling road I use is good and does a lot of race cars.
We tried everything to remove the stumble except change aux venturi and chokes.
Unfortunatley the rolling road I use is about to change hands and I may try another one in Cambridge.
But I don't want to go to all the bother of another session if I end up with the same result.
Lister

[listernoble]

"Did you have your cylinder head tested on a flow bench ? its not uncommon for the gas flow to have glitches in it as valve lift and gas flow increases ? . . .this could then be attributing to the drop off at 5k ? I'd have thought a fuel or ignition problem would have been sorted on the rolling road, the exhaust probe should tell you what the mixtures doing ??? have you e set and checked the fuel pressure "

Head is standard Carter full race head so it will not be that.
Fuel pressure is fine at all other speeds so probably should not be that?
I am waiting to hear from pals re jetting etc.
Lister

[coln72]

Just a way out theory. Are you having some sort of harmonic problem, as is at that rev point you are experiencing vibrations that may be upsetting the fuel flow in your carbies?

[listernoble]

No,the engine runs smoothly all the way up,I have spoken to another engine tuner and he reckons that there is over fuelling causing the bog down.
Apparently some of the short oval hot rods get over the same trouble by modifyiing the accelerator pump and I have booked a session with him later this month.
I am away on holiday for a few days at the weekend and will update the forum after the tune up session.
But please keep any thoughts coming.
Lister

[zefarelly]

Originally posted by listernoble
[BHead is standard Carter full race head so it will not be that.
Fuel pressure is fine at all other speeds so probably should not be that?
I am waiting to hear from pals re jetting etc.
Lister [/B]

out of interest, did you get a flow bench chart with the head ? I prepared my own head and then got it flowed . . . .it needs more work, although I'm more inclined to do another one to avoid the chance of making it worse ! but having paperwork to back stuff up is always a good idea, and will help no end when encountering these kind of problems, even if it just eliminates the obvious

[listernoble]

The head is probably around 30 years old and Carter would not have had a flow bench.
His engines are the best Imp derivatives around so it almost certainly will not be the head.
Lister

[graham bahr]

are your carbs mounted correctly? if they are mounted solidly rather than on thackary washers they can cause flat spots.

are your valve springs ok? it might sound a silly question seeing as your engine will rev on well past the flat spot, but if one spring has gone soft or maybe broken an inner one it could be hitting a bad harmonic at that point in the rev range and causing slight valve float at that point which goes away as the harmonics change as the revs rise, just in the same way that an exhaust can vibrate and cause an annoying noise at just one particular rpm.

having said all that its probably a distributor problem such as a worn base plate causeing timing scatter, or maybe the distributor/oil pump drive is worn causing timing scatter, if i remember rightly high rpm imps had to have one of the crank/ oil pump drive gears changed for a bronze one to stop wear and timing scatter, maybe after 30odd years its now worn and at fault.

[zefarelly]

Originally posted by listernoble
The head is probably around 30 years old and Carter would not have had a flow bench.
His engines are the best Imp derivatives around so it almost certainly will not be the head.
Lister

no offence or disrespect to you or Carter, I'm sure his head is better than mine !!!

my head is 40 years old, by spending £100 I got it slapped on a local flow bench just to get some figures, it told me 2 invaluable things, firstly that theres no pont in lifting the valves high as it doesnt improve flow (the chart goes flat after 1/2" lift) and secondly, there was a glitch in the flow on the exhaust side at about 8-9 mm lift.

now from talking to all sorts of people, you can do 2 heads that look, feel, measure and weigh exactly the same, but they'll flow differently, equally, what looks like a great piece of porting may well flow worse than the factory item . . . .for a relatively little outlay you can guarantee that your head is doing what it looks like it should, and eliminate time and money on the rolling road, let alone rebuilding the top end.

It is a very dark art that people wont discuss, but its very simple to ensure your going forwards with developement and not backwards.

for what its worth, I reckon your problem lays elsewhere, as others have already suggested, carb jetting, and timing.

[graham bahr]

having said all of the above,

it still could be your cam, my car has a similar dip in the power curve which is the cam ( i run full engine management on fuel and ignition so wobbly distributors and dodgy jetting arn't an issue.) but unless i cock something up its never an issue for me as the revs are always above it, on my engine power drops 20 brake from 4500 to 5500 then from 5500 to 6000 it leaps up 40 brake before finally peaking at 7800 its really noticable in the way you describe if i do drive through it.

are others that are using the same cam also running the same gearing and tyre sizes? it might be that it doesn't notice on their cars, it would seem to me that your cars geared too tall, considering how high up the rev range peak power is.

[listernoble]

We have just lowered 1st and 2nd ratios,max speed in first is 42mph at 9000rpm,car max speed in top 103 at 9000rpm.Bear in mind it's a hillclimb car,if it was a circuit car there would not be a problem.
After my hols we are going to put it on some rollers near Cambridge and play around with the pump jets and action,as the operator reckons he can probably cure the bog down.
He reckons it is over fuelling just before it comes on the cam.
Also we will try 30 mm chokes instead of 32's and play around with the other jets,because checking with pals I'm running more fuel.
It's worth a try.
Lister

[zefarelly]

Originally posted by listernoble

we are going to play around with the pump jets and action,as the operator reckons he can probably cure the bog down.
He reckons it is over fuelling just before it comes on the cam.
Also we will try 30 mm chokes instead of 32's and play around with the other jets,because checking with pals I'm running more fuel.
It's worth a try.
Lister

this is what mine was like before RR . . . .snotty, then at 3500 your off . . . .changed pump jest and reduced mains, job done.

good luck and keep us posted . . . .10/10ths sohuld publish a book of tuning problems solved, then we'd never have a problem engine amongst us again !!!