Engine Dead Spot

[paw140]

After nearly 3 years, I’m finally able to drive my 1969 S4. The engine is freshly rebuilt. It runs mostly ok, but it does seem to have an issue at very light throttle from 2000-3000 RPM. If I try to lightly accelerate at this engine speed, the engine sputters and is very choppy. If I give it a little more throttle, everything clears up at it accelerates strongly. I’m thinking it’s a carb tubing issue, but mine is equipped with the federal Strombergs with needles that cannot be adjusted. Does anyone have any advice?

I pulled the spark plugs and this is what they look like after 120 miles. #4 is very clean, almost looks new. The others are a little dark, but I’m thinking they are ok. I’m puzzled by #4… I’ve seen it suggested that this could indicate a small coolant leak, but I’m not seeing any evidence of that.

Thanks!

Paul

[paw140]

Picture of plugs:

[mbell]

Probably a carb issue but I'd try bpr6s plugs. As might self clean better and help improve running.

[joe7]

#3 looks like its cracked.

[paw140]

Yes, I broke #3 when I was removing it.

Does anyone know how I could convert my carbs to use adjustable needles?

[nmauduit]

are you sure that #4 gives consistent spark ?

[USA64]

I'd start with a thorough review of the carbs. The emission Stromberg have (had) a lot of bells and whistles related to emissions. Do you still have the cross-over piped? Are they functional? They are bypassed at higher throttle openings using a secondary throttle. Is that in place? Does it function well? How about the temperature compensator. Probably some or all of the little tubes are capped off. Is your vacuum advance hooked up? And so forth.

[Sadbrewer]

Yes, I broke #3 when I was removing it.

Does anyone know how I could convert my carbs to use adjustable needles?

You don't need it Strombergs are reliable, you just need to get a competent mechanic to set them up..owners messing about with carbs is not a great idea for the smooth reliable running of any car.

[paw140]

I’ve removed the crossover pipes and the secondary throttle plates. I’ve wondered about the temperature compensators…

[baileyman]

These use oil dampers? I used to put thicker oil in mine when the throttle response deteriorated. John

[paw140]

I have found a hint to my problem. It is very hot today (32 C / 90 F) and I drove the Lotus and the hesitation is pretty much gone. I also noticed on previous drives that the problem got much less noticeable when I drove the car, let it sit for an hour, and then drove it again. I also noticed the problem was quite bad when I drove it on Friday and the temps outside were 50-60 F. I’m suspecting my temperature compensators are not adjusted properly, causing it to run too rich when cold. I’ll take a look at that in the coming weeks.

[paw140]

Update:

Since my old distributor was worn out anyway, I decided to buy a new Pertonix unit. It made the problem slightly better, but at least confirmed to me that it wasn’t an ignition problem. I also installed a hotter thermostat since the one that RD sold to me seemed a bit cold (I switched from 160F to 180F). Both of these things seemed to improve things a little but definitely didn’t fix the problem.

Then I tried something that made a huge difference - I took the top of the carbs apart and moved the needles about 0.020” higher, about as far as I could go, to richen the mixture. The tick over quality was so much better and I took it for a drive and I’d say that the dead spot is 90% gone. It runs a lot better.

Reading more on this forum, I understand that the federal non adjustable Strombergs were tuned very lean to meet US emissions regulations. I also found a reference to an old Lotus Tech Service bulletin about adjusting the depth of the main jets with a special tool because many times they are too lean from the factory. I see that I can buy adjustable jets, which I may install over the winter.