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engine stutter of rebuild BT7 engine [tri-carb] above 2500 rpm

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mengels

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Hi All,
In need a solution for the engine stutter I have above 2500 rpm (with warm and "just warm" engine). The engine picks up great and runs great under 2500 rpm, above 2500rpm it starts to stutter a bit. The engine has been rebuild 2 year ago. What I've done so far for a solution to my problem:
- checked rotor and distributorcap of the CSI electronic ignition for oxidation and loose parts
- new spark leads and coil to distributor lead
- new Champion N12YC sprak plugs
- cleaned fuel filters and checked fuel volume at carb end, it fills a 30cl beer bottle within 10 sec or so
- checked ignition coil, primary 3.5 Ohm and secundary 10 kOhm

Any suggestions what could be the problem or where I can look to solve this problem?

Thanks!

Maarten
(The Netherlands)
 
Your symptoms sound like an ignition problem, but you've checked all the 'usual suspects.' Someone is going to tell you to put points in the distributor, so I might as well, but that may be difficult or impossible with your electronic system. The only other thing might be an intermittently bad plug, but I've never had an issue with the same Champion plugs and you'd either have to swap one at a time or all at once to rule them out.

What kind of secondary spark plug wires are you using? Did you check the impedance of them and the coil wire from end-to-end? Stranded copper wires should have almost no impedance, the coil-wound 'suppression' type probably have some, and using them with resistor plugs could cause a problem. What gap did you set on the plugs?

Did you check the primary wiring to the distributor? Sometimes, the fragile little flexible wire to the points base plate can be intermittent, but I would expect that to cause complete stalling. Is your distributor advance plate free (or, did you get the 'no advance' version of the distributor?)? I think you'd want some advance, in addition to the mechanical, even over 2,500RPM at partial throttle.

Inhibited fuel delivery usually causes a serious stumble, not a stutter, and SU pumps--or do you have aftermarket?--are usually all-or-nothing, more-or-less, when they start to fail.
 
I had a similar issue a few years ago. It was a problem with the plastic right angle caps on the spark plug leads which attach to the plugs. The caps had an internal piece of carbon that broke down and caused a high speed miss. I think the plastic caps were stamped with the word Champion on them. I replaced the leads with a different set and the issue went away.
 
My thought is that you may not be getting sufficient ignition advance. With the vacuum advance disconnected, do you get 35 def of advance at 3,500 rpm?
 
Hi,
I replaced all the spark plug with brand new N12YC ones (gap checked at 0,6mm), all spark plug leads (with the lead from the coil to the distributor cap) are replaced with a lucas set from AH Spares UK.... Just checked the coil, with its primary at 3.5 Ohm and its secundary reading at 10 kOhm, so that seems not to be the problem either...
Is it possible that the issue is caused by some debris in the carburettor bowls because of the car has been sitting still for about 8 months or so...that is the only item I did not checked yet....
 
Hi John,
An advance problem could certainly be possible, What do you mean with "do you get 35 def of advance at 3,500 rpm" (I'm not a native speaker... ;-) )
Do you mean 35 degrees of advance? How can I check the actual advance with the engine revving at 3500 rpm?
Thanks!
 
Yep, I was thinking fuel starvation. Also, possible that the carbs are set too lean.
 
mengels said:
Hi John,
An advance problem could certainly be possible, What do you mean with "do you get 35 def of advance at 3,500 rpm" (I'm not a native speaker... ;-) )
Do you mean 35 degrees of advance? How can I check the actual advance with the engine revving at 3500 rpm?
Thanks!
Click to expand...
The ignition timing should advance as engine speed increases so that maximum cylinder pressure is reached at top dead center (TDC).

If you have the original crankshaft pulley, there is a notch cut in it. When that notch lines up with a pointer on the timing chain cover, piston #1 or #6 will be at TDC.

If you are using a stroboscopic timing light, it will flash when the plug for cylinder #1 fires. At 600 rpm, the plug should fire at 15 deg Before TDC (BTDC). 15 deg BTDC is approx 13/16" to the right of the notch on the pulley. As speed increases, the timing advances and you will see the notch appear to move to the left. If that isn't happening, your distributor advance isn't working. The maximum advance is 35 deg BTDC. 35 deg is approx 1 7/8" to the right of the notch on the pulley. The RPM where maximum advance is reached is about 3,000 to 3,500 RPM. When measuring this, the vacuum line to the distributor is disconnected.
 
Check then CSI ground wire and wire terminals on the distributor. Check ground strap tonengine and the ground to the distributor.
 
Thanks all for your input, there are certainly some points I can look into....I'll keep you posted for results (also for future readers with similar problems...)
 
Maarten,
Replacing spark plugs and testing them one at a time is good advice. I once had a defective Champion spark plug that was brand new. It caused a similiar problem.
Then, you might replace the coil with another that is known to work properly. They can fail as they get hot.
Good luck.
Douglas
 
dcarlg said:
Maarten,
Replacing spark plugs and testing them one at a time is good advice. I once had a defective Champion spark plug that was brand new. It caused a similiar problem.
Then, you might replace the coil with another that is known to work properly. They can fail as they get hot.
Good luck.
Douglas
Click to expand...

Hi Douglas,
Thanks for the tip. I already changed all the (good working) spark plugs with new Champion plugs as one possible solution....(without succes...). I think it's unlikely that I have one (or more) faulty spark plugs in the 2 sets I have.....
 
Next to all the things I already tried and tested, I also swapped the spark coil with a new Blue Bosch and connected the ignition system directly to the battery to bypass any possible dodgy connection. Nothing gave the solution I'm looking for. I contacted the manufacturar of the CSI electronic ignition and they will test the electronic ignition on their test bench...
I'll will report the findings of the test shortly....
 
Is the rotor arm a really tight fit? I had a similar hesitation under hard acceleration and it was due to a loose fitting rotor. Get a new one from Distributor Doctor. Worth buying even if it doesn’t solve this particular problem. Also check the fu el pump is delivering sufficient fuel at higher revs.
 
Check your carbs again, I run HS6's as standard on my BJ7 but when I first had the car I could not get above 60 MPH, I eventually narrowed it down to a blocked carb.

What colour are the spark plugs after the stuttering occurs? A couple of years ago I had a problem with over fuelling that also caused stuttering. Plugs were black with soot, new needles and jets sorted it out.

:cheers:

Bob
 
Bob Hughes said:
Check your carbs again, I run HS6's as standard on my BJ7 but when I first had the car I could not get above 60 MPH, I eventually narrowed it down to a blocked carb.

What colour are the spark plugs after the stuttering occurs? A couple of years ago I had a problem with over fuelling that also caused stuttering. Plugs were black with soot, new needles and jets sorted it out.

:cheers:

Bob
Click to expand...

Hi Bob,
I'm afraid I walked the same path as you did and come to the conclusion that it must be the carbs, although I cleaned them thoroughly....the plugs of the two cylinders nearest to the firewall (I use this description because the description "cylinder #1" is sometimes food for discussion) are more black, but until now I put that down to the fact that the venting tube of the valve cover is connected to the air cleaner house of that particular carb. Maybe it's set to rich or the choke lever is hanging....I'll check again!
 
Mengels,
Suggest measuring the jet drop from the bridge on each carb then averaging them out.

Remove the domes with their pistons (being careful to keep each piston with its matching dome). Then take the tail of a dial caliper and measure how far down each jet is from the bridge. You may find the rear one different from the front and middle. Then you can average them out to the same drop for all 3.

For 1.5" SUs, jet drop should be around 1mm/.040"*.

Edit: *Hammill, SU Carb High Performance Manual, p.44. Starting point for tuning.
 
Last edited:
OK, the problem is solved...!
I noticed that the engine was running perfectly through all rpm's when stationary (so not under load), but the problem returned when driving..... I knew then that I could discard the ignition part as source of the problem.
When driving without the air filters the problem was gone....inspection of the airfilters showed that the airfilter of the carb closest to the firewall was partly blocked, making the mixture rich under load, because it was partly saturated by oil of the valve cover vent hose which was connected to it....I did not saw it before because the airfilters are black which camouflaged the oil....
Thanks for all the input!

Maarten
 
If you're getting that much oil in your rear air filter you should check your rocker shaft for wear. If it's spouting oil you're probably losing a PSI or two of oil pressure.
 
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