Early Brass Marvel woes, '15 Buick

I am wondering if anyone has had experience with early Marvel carbs? I have adjusted the primary jet and the engine runs smoothly but when I open the throttle it runs lean (pops back through the carb), regardless of where the air adjuster screw is set. I can make it better by adjusting the primary jet screw (richer) but then it won't idle at all well. It seems to me that I should increase the main jet size as various springs I have tried don't solve the problem, although they do influence it.

Any ideas appreciated as I've spent all day on it and it is no better than when I started !!

Regards

Al

Mark Shaw & Dave Brennen aka DandyDave are among the best. They can help. Just PM them.

It is pretty easy to drill out the main nozzle to give you a little more fuel. Get a set of numbered drills between 60 and eighty. By using these bits determine the size. Usually two or three sizes larger will do the trick. If you get the hole too large, silver solder and redrill the hole. The alcohol in today's gas wants a larger nozzle.

Mark Shaw & Dave Brennen aka DandyDave are among the best. They can help. Just PM them.

Hi Larry,

I have contacted both and they have both responded, thanks.

Al

Al Brass: when you figure out what the 'trick' was to make it run better, please come back here with a message sharing what the 'fix' is.

Thanks

GLong

Al Brass: when you figure out what the 'trick' was to make it run better, please come back here with a message sharing what the 'fix' is.

Thanks

GLong

I'll second that. My 1926-26 Buick has a similar issue. Keep us posted.

Hi Guys,

I now have it running much better and I am about 95% happy. Mark and Dave offered some advice but I know it is difficult when you're not standing beside the car. After countless secondary jet changes and enlarging the jet to as much as a #70 drill and then trying each of the three different springs I had, I couldn't get it much better. When I opened the throttle, it hesitated, popped back and sounded flat. At speed, it still popped back through the carb. I could minimise this by altering the primary jet needle but then it wouldn't idle well. Very frustrating and much tail chasing went on. In the end, I gave up and decided to modify.

I fitted an adjustable needle to the body, directly under the base of the secondary jet tube and removed the jet. I wasn't sure how the reduced height (with the 1/4" of jet missing) would work but it seems fine and so what I now have is an adjustable main jet.

I fitted my medium spring and wound the spring screw right it.

I started the engine and adjusted the primary idle jet to obtain a smooth idle.

I opened the secondary needle about a turn.

I opened the air screw until it was level with the ratchet spring as described in the manual.

I cracked the throttle open but there was still some leaness.

I adjusted the hand throttle to a medium engine speed and opened the new screw a little until the engine sounded smooth.

I opened the air valve screw a little further to get an optimum setting.

So, the secondary (main) jet tube is now a discharge tube with adjustable metering done at the base. Time will tell if it is successful and I don't really like altering something that my mind tells me used to work but after two days work and no progress, I had to do something. I trust I have explained everything well enough but please don't hesitate to contact me if I have confused you or not explained myself clearly.

Regards

Al

I hope that works for you. In my limited experience with these carbs, I have found success by getting the main circuit dialed in first, regardless of its effect on the idle. Then fix the idle without altering the main. The reason (at least according to my oft faulty logic) it that the main adjutment has a more dramatic effect on idle quallity than idle adjustment does on high speed power.

So afer all the basics are covered, (like clean, no worn out parts, proper fuel supply) get the main set for drivability, then figure out why it won't idle. What I have found before is either a vacuum leak, or throttle blade not closing enough, thus requiring wacky idle mixture adjustments to compensate. In either case, you end up with a too rich or too lean setting on the idle mixture to attain a proper idle speed, but then suffer all the problems asociated with running too rich or too lean.

I hope maybe this helps someone a little. Good luck. Joel

Hi Joel,

What you suggest makes good sense but the reason I set up the idle first is that the primary jet contributes to fuel delivery right through the rev range. I have never found a nice idle difficult to find, it is the power performance that has tried my fortitude !!

Regards

Al

Al, does your gasoline in New Zealand have ethanol mixed in it? We have lots of problems with Ethanol tainted fuel in the states, Very low compression engines don't burn ethanol well if any, the ethanol is just a 'filler' in the gasohol. So we have to re-jet to compensate for the 'filler' in the fuel.

I've found that I need to find ethanol-free gasoline in order to get early engines to run well without re-jetting the carb.

Glad you got your car running better, regardless of what it took to accomplish it.

GLong

Did you adjust the float to carry a higher gas level in the fuel bowl?

Hi cahartley,

I tried adjusting the float level up and down but it didn't make too much difference. I went back to the factory setting of 1.25", just below the primary jet.

Glong, we don't have ethanol in the South Island yet but The carb last ran properly on leaded gas. We can't buy that at the pumps now and maybe this is where some of my problems originate.

Regards

Al

The spring on the left is the one you should be using:

http://www.thecarburetorshop.com/Marvel_springs_HR.jpg

Popping through the carb could also be a sticky intake valve, or valve timing.

Jon.

Hi Jon,

The one I am using looks like your one on the left. The problem is I have three, all different lengths but seem to have the same number of coils and made from the same gaged wire. Can you help with a new spring please?

The engine is now running pretty sweet, no popping back at all but I would obviously fit a new spring if I could get one.

Regards

Al

Does anybody have the correct main jet size for a Marvel as fitted to a Buick Model C24 or C25 ? I have found that there is a huge variation in the ones I have but I do not know what their application is. They may not even be Buick.

Regards

Al