Stuttering and misfire likely cause?

[richberman]

So now that I've got 50 miles on my car I've noticed that the car feels like it stutters a bit when it goes uphill or needs power. It also misfires occasionally, mostly in the same situation. Timing is set at 14 degrees advanced and gunson color tune helped set carbs to half way between yellow and start of leanness slowing idle like instructions state. I've retightened exhaust and intake manifold bolts. What is the likely cause. Btw it's a tricarb HS4 setup.
thanks for the help.
rich

 

[Bob_Spidell]

Had same symptoms due to a leaky distributor cap. Try the 'dark test:' run the engine in a dark but VENTILATED garage and look for arcing around the distributor or plug wires.

 

[57_BN4]

Hi Rich,

What did you do to the fuel tank, is it possible dirt is coming through and causing issues? Also, what needles are you running?

I had a definite leanout under power after swapping out the dud (lumpy) cam because the manifold vac is now so much greater. Cured by putting the needles in the battery drill and polishing with some fine sandpaper and measuring them constantly to keep the profiles the same. It doesn't take much polishing to change the mixture strength. Idle mixture is not related to power mixture, other than by the needle profile.

Pull out the choke a little when it is misbehaving- if it gives any improvement then you might need some richer needles if everything else is in check. Andy.

 

[nevets]

might be the coil...I had similar symptoms and turned out to be the coil.

 

[richberman]

Had same symptoms due to a leaky distributor cap. Try the 'dark test:' run the engine in a dark but VENTILATED garage and look for arcing around the distributor or plug wires.

Bob,
I'll try that, but dizzy, coil and wires are all new and/or rebuilt by Jeff at Advanced distributors and it no longer misfires at idle since I adjusted with colortune.
rich

 

[richberman]

Hi Rich,

What did you do to the fuel tank, is it possible dirt is coming through and causing issues? Also, what needles are you running? Fuel tank was cleaned and red-coated. Also added fuel filter.

I had a definite leanout under power after swapping out the dud (lumpy) cam because the manifold vac is now so much greater. Cured by putting the needles in the battery drill and polishing with some fine sandpaper and measuring them constantly to keep the profiles the same. It doesn't take much polishing to change the mixture strength. Idle mixture is not related to power mixture, other than by the needle profile.

Pull out the choke a little when it is misbehaving- if it gives any improvement then you might need some richer needles if everything else is in check. Andy.

Andy,
Carbs and throttle linkage was all overhauled by Tom Monaco and I'm not certain of the needle type. I'll ask him. I'll try the choke to see what happens. Maybe I can just open up the carb (rear one) a flat each time it happens until it stops???
rich

 

[steveg]

Bob,
I'll try that, but dizzy, coil and wires are all new and/or rebuilt by Jeff at Advanced distributors and it no longer misfires at idle since I adjusted with colortune.
rich

Rich,

I had similar misfiring with newish Moss gold coil. Car would miss for a block or so on starting off from a stop sign in 80 degree weather. I originally blamed it on ethanol in the gas.
Switched to Pertronix unballasted coil and problem went away. Jeff recommends Bosch blue coils - is that what you're running?

Did you try swapping out the condenser? Jeff tests the condensers he sells, but they're still the same poorly made design and may fail after a while. If you still have your original condenser that might be better.

Also try disconnecting trunk cutoff switch wire from coil.

PS - remember most "fuel" problems are electronic!

 

[DerekJ]

Also check plugs for gaps etc. What type of plug are you using? What heat? I just changed my plugs and it stopped the spit back from my Webers. I check the plugs regularly and thought they were Ok but evidently not. Also I'm wondering if your ignition might be too advanced for the rubbish fuel that we now have. Do you know what the advance is at 3500 revs. I have a high performance engine but I'm only at 31 degrees full advance.

 

[richberman]

Also check plugs for gaps etc. What type of plug are you using? What heat? I just changed my plugs and it stopped the spit back from my Webers. I check the plugs regularly and thought they were Ok but evidently not. Also I'm wondering if your ignition might be too advanced for the rubbish fuel that we now have. Do you know what the advance is at 3500 revs. I have a high performance engine but I'm only at 31 degrees full advance.[/QUOTE

NGK5BPES plugs
14 degrees advanced at idle. Not sure about 3500 rims. That would require two people.

 

[Randy Forbes]

I've always used BP6 ES (NGK) plugs; I've done tests__on my own car only__using the 5s and 7s and the 6s were the best choice.

Three HS4s is a nice combination, but on what engine? I know, IF I had built it, I would know! I put the same setup on my original 100/6 2.6 litre engine, and with stock__3000__needles, it was too rich. If you're running a 2.9 engine, the stock needles would be fine, just down to fine tuning and the ALL IMPORTANT synchronization of the throttle openings. A stumble on acceleration is usually a lean indicator__back-firing on the overrun is normal.

I think my total advance is in the region of 32*; in any case, less than 34* (haven't touched since I moved__only change being that I had to increase the low speed fuel flow).

Drop by sometime, now that you're driving it, and I'll check the timing and carbs for you

 

[DerekJ]

Randy, I'm also using BP6 ES. Previously I had B7 ECS as the engine is for competition. However the competitions are only hill climbs of short duration plus I also do a lot of road driving. I found the 7s were too cold and the plugs were tending to oil a bit. The 6s seem to be working perfectly- fingers crossed.

 

[steveg]

FWIW - I'd used the BP5ES a little bit 10 years ago; the BP6ES work really well - always the proper color. Am assuming if a colder plug shows good color, better to run the colder plug.

 

[richberman]

I've always used BP6 ES (NGK) plugs; I've done tests__on my own car only__using the 5s and 7s and the 6s were the best choice.
I was mistaken about...I did use 6s...based on your experience.
Three HS4s is a nice combination, but on what engine? I know, IF I had built it, I would know! I put the same setup on my original 100/6 2.6 litre engine, and with stock__3000__needles, it was too rich. If you're running a 2.9 engine, the stock needles would be fine, just down to fine tuning and the ALL IMPORTANT synchronization of the throttle openings. A stumble on acceleration is usually a lean indicator__back-firing on the overrun is normal.

I think my total advance is in the region of 32*; in any case, less than 34* (haven't touched since I moved__only change being that I had to increase the low speed fuel flow).

Drop by sometime, now that you're driving it, and I'll check the timing and carbs for you

I would love that if I could guarantee it could make it to Florida!

Update: it turns out the mis-firing and stalling was a breaking wire between the points and external (white wire) bolt. I must have overtightened the white wire to the point of kinking the internal wire into the plate.
I made it on a 2 hour drive and it only stutters and slightly misfires at around 3200-3500 rpms. Tomorrow I'm planning on re-tuning it to see if it can be improved. I'll check for vacuum leaks too.

 

[57_BN4]

Hi Rich,

Have a read your plugs after the run, your stuttering sounds just like mine was before I 'reprofiled' the needles. Turning the idle mixture screws won't affect the higher rpm mixture so if it is running lean then your only option is changing needle profiles. Even pulling the choke out doesn't really compensate for a lean mixture because pulling the jet down also ruins the venturi effect to some degree. Andy.

 

[richberman]

Hi Rich,

Have a read your plugs after the run, your stuttering sounds just like mine was before I 'reprofiled' the needles. Turning the idle mixture screws won't affect the higher rpm mixture so if it is running lean then your only option is changing needle profiles. Even pulling the choke out doesn't really compensate for a lean mixture because pulling the jet down also ruins the venturi effect to some degree. Andy.

Andy,
I'll post my results after checking everything else out tomorrow.
thanks for the thought on the needle profiling.
rich
P.S. what kind of specs are we talking about?

 

[57_BN4]

P.S. what kind of specs are we talking about?

SU needle profiles are a whole industry... Back in the 90s when I worked at a garage offering dyno tuning there was a big leather strap with about a hundred needles stuck through it to "suck it and see". Probably easiest to find somebody locally that specialises in SU carbs and try swapping a few needles with them. Alternatively you can buy needles or do what I did and put them in a drill/sandpaper which isn't very scientific at all.

Somebody has gone to great lengths on this website https://www.mintylamb.co.uk/suneedle/ to capture all the needle profile data into a spreadsheet which graphs needles together so you could find your current profile and choose one that is a bit richer in the middle section. Could get expensive though.

If your engine is built with performance driving in mind then get either a dyno tune or a wideband O2 sensor and gauge, or if you are only cruising then reading the plug colours will be accurate enough. Tons and tons of info on Google for SU carb tuning, everyone has a favourite method.

Andy.

 

[johnea]

Misfiring at 3000-4000 RPM's may also be caused by a condenser not making good earth.

 

[steveg]

Misfiring at 3000-4000 RPM's may also be caused by a condenser not making good earth.

The condensers being made now are internally flimsy and have a poor internal ground mechanism that is subject to fatigue. A manufacturing company I consult with has analyzed them.

I've discussed this problem with Moss' head of quality control (BJ8 owner; member of SoCal Austin Healey Assn). He says it's a worldwide problem for all cars using now-obsolete condenser-points ignition. Moss are working on sourcing old-style good quality condensers, but it's an extended problem.

In the meantime, if one couldn't get a NOS condenser, the best thing would be to buy a half-dozen of Jeff Schlemmer's tested condensers and keep swapping them out every time the problem occurs.

FWIW - Welch sells a high-quality "racing" universal condenser that attaches to the coil. The engineer at the company mentioned above said that one will cause radio interference due to it's distance from the points.

 

[richberman]

So part of the problem was a vacuum leak at the balance tube. Apparently a misfire in the third carb blew the balance tube upwards causing the leak. Now I'll need to to back over the tuning again to see if that solves it. Waiting for new ground wire though.