Replacing a Mikuni Carburetor with a Weber Carburetor

[SubGothius]

By "static/base" do you mean loosen the distributor and rotate it slightly clockwise a bit? I don't have a timing light, so I'd have to just tweak it and see what happens.

Yeah, that's it -- how much advance is dialed-in for the dizzy itself at idle, before centrifugal/mechanical advance and vacuum advance kick in. BTW, your local AutoZone / Advance / O'Reilly's may have timing lights available for loan/rental.

[geezer101]

Try dynamic tuning as well. You'll need a dwell/tach meter to observe engine RPM's. This is a relatively straight forward procedure. Reset the plug gaps as I've recommended earlier and have 91+ in the tank (you'll be tuning the engine to run on a higher octane fuel and may need to re-tune if you go back down to regular pump gas) Start the truck, loosen off the distributor retaining nut so you can adjust advance/retard. Hold engine RPM @ 2000 and gently adjust the engine rpm between advance and retard until you hit a sweet spot and engine RPM's jump by themselves, then lock the distributor retaining nut. Take it for a drive and see what it's doing as you may need to make further adjustments.

*timing light is not much more than a diagnostic tool to see if you've getting misfire or some event is causing timing fluctuations. If you've set the carb throttle stop and have cold and hot idle RPM's right, you can tune the engine by feel.

[Chairwolf]

SubGothius and Geezer101,

Yesterday I removed the distributor cap and checked the vacuum advance by sucking in on the vac. tube. The advance would move, but would not hold its position, so I believe the valve is leaky. I have a new one ready to install. It might be a few days before I get that done and have a chance to check the results. Will also re-gap the plugs.

This truck needs other things done to it! I took it for a ride yesterday, and noticed that the steering wheel rotates left and right a little at speeds above 40mph. So it needs some front end work! This will be a lengthy project vehicle! But thanks for your advice, I will work on these suggestions and report back as soon as I can!!!

[Chairwolf]

Yeah, that's it -- how much advance is dialed-in for the dizzy itself at idle, before centrifugal/mechanical advance and vacuum advance kick in. BTW, your local AutoZone / Advance / O'Reilly's may have timing lights available for loan/rental.

I'll probably do that, or maybe check Harbor Freight. I don't mind buying a tool, but would rather not buy something that I'll probably not use much if at all!

[geezer101]

When you've installed the new vac advance, give the mounting platter a good blast with WD40. They gum up and it causes 'lag' - won't respond on quick open throttle and doesn't release fast enough off throttle.

[Chairwolf]

Ok, the vacuum advance I received was for a 2.6, not a 2.0. So I returned it, and ordered a full distributor with the advance on it. Should be here by the weekend. No other updates so far!

[Chairwolf]

Ok, the new distributor is installed and the truck runs. Still has difficulty starting, but I found a video channel by a guy in South Africa who knows Webers very well. He lists 5 things to check. I think my problem could be either distributor advance, or vacuum leak. Here is the video and his channel, which seems to be excellent: https://www.youtube.com/watch?v=t-VCi1lS8vU

Any thoughts?

[Chairwolf]

I think my next check will be running the engine and using carb cleaner to try to find vacuum leaks.

[Chairwolf]

Well, fooey! After I put in the new distributor, it ran fine. I re-gapped the plugs to .80 (highest gap I have on my gapper) and it wouldn't start, so I went back to .44 and it ran. Next time I tried (few days later), it wouldn't start. I haven't been able to start it since. I put in a new rotor, adjusted the distributor, cleaned the plugs with a wire wheel. Still won't start. I'm thinking air, fuel, spark. I have air, it's getting fuel, it has spark. Timing *seems* to be right. I tried my old trick, taking a plug out, plugging it to the wire, and setting it close to the cylinder hole, but not inside. Then crank. When it is time for the spark and power stroke, it ignites the air/fuel mixture and pops, so I know I've got air, fuel, and spark. (Also cures flooded cylinders!) So, now I'm thinking carb. The truck has been sitting for about 5-6 years. Maybe I need to rebuild the carb. Any ideas?

[geezer101]

Reduce your plug gap and adjust your advance timing. You will need to aim for 0.85mm/0.9mm to get a good, aggressive arc (.033/.035 in) The Weber requires more aggressive timing and is sensitive to engine vacuum.

[Chairwolf]

Ok, update time! I gapped the plugs as recommended to between .033 and .035. Adjusted the timing, also added a missing ground strap. Still refuses to start. I ordered a weber rebuild kit and a jet kit. I'm thinking that maybe the 5-6 years of not running has allowed the ethanol gas to gum up the works. I'm also thinking I should confirm that the coil is working. I will also confirm good spark. I've replaced the distributor and rotor, and also CLEANED the plugs. I may put in new plugs and wires, and if needed, coil. That should cover the spark question! So next, confirm good carb, then I may drain the gas (If I can figure out how; siphon hose wouldn't reach the gas. Any tips?).

I'm open to suggestions! Thanks

[geezer101]

I think you need to drop the tank out and flush it. Siphoning will only get so much and I've had experience with a tank that was full of silt. First time I got the car on the road it pulled a ton of junk up into the lines and killed it.
It will also give you a chance to inspect the overall condition of the tank and maybe encourage you to give it a coat of paint for future protection - and it looks pretty

[Chairwolf]

I may do that, but later on. My to-do list is already getting pretty long! I'm hoping to get it running and burn most of the old gas up.

[Chairwolf]

Ok, another update. I got her running! Here's what I did:

- I connected a manual fuel pump to the fuel line in the engine compartment and pumped the old gas out, and replaced it with fresh gas.
- Rebuilt the old Weber with the official kit.
- Set the crank to TDC and got the distributor timing right.

and now she runs! It idles fast, and I'm having a little trouble with my throttle cable linkage coming loose. I have the automatic trans, so I need the kick-down linkage. I may end up putting the original throttle connector back on, but as I recall, that got in the way of the choke linkage. We'll see.

So, current issues:
- throttle cable coming loose
- idles great, but dies under load.

Any suggestions or threads you can point me to where these things have already been hashed out?

Thanks!

[geezer101]

It will need to idle fast if you have an auto trans - how fast is it? You are going to have to experiment with ignition timing to run the Weber. Did altering the plug gap make the difference?

[Chairwolf]

It will need to idle fast if you have an auto trans - how fast is it? You are going to have to experiment with ignition timing to run the Weber. Did altering the plug gap make the difference?

At present, the gap is .033. I noticed when I did the rebuild that the "full power valve diaphragm" had a little pucker in the rubber. After doing the rebuild, and some more googling, I learned what that thing did, which is to enrichen the mixture at full power. So maybe that's the problem. I ordered a new one from PegasusAutoRacing.com. I'll put it in when it gets here and see if it helps.

When I attempted to drive it, it stalled as soon as I put it in gear. When it would try to go, it rattled. Do you think the valve will fix this, or maybe advance the timing a little more, or something else? I may also need to re-check the float settings. This is my first time messing with a carb to this level, so forgive my "newbie" questions! If it sounds like I'm not familiar with the weber, it's because I'm just learning.

[geezer101]

It sounds like timing...? If it's rattling straight on throttle it seems like it's over advanced. This is an awful way to adjust idle, but back off the advance to drop idle speed and see what it does. Normally to set idle you'd warm it up, set the throttle stop and then adjust timing. The thing with the Weber is the added vacuum it produces - it automatically tries to pull vacuum advance straight from the get go.

[SubGothius]

I just feed manifold vacuum to the advance, ever since I found out the only reason for ported vacuum (taken from the carb just above the closed throttle butterfly) is to reduce emissions at idle by raising combustion temps. Manifold vacuum applies full advance at idle, reducing combustion and engine temps.

[Chairwolf]

It sounds like timing...? If it's rattling straight on throttle it seems like it's over advanced. This is an awful way to adjust idle, but back off the advance to drop idle speed and see what it does. Normally to set idle you'd warm it up, set the throttle stop and then adjust timing. The thing with the Weber is the added vacuum it produces - it automatically tries to pull vacuum advance straight from the get go.

I'll give it a try! Thanks

[Chairwolf]

I just feed manifold vacuum to the advance, ever since I found out the only reason for ported vacuum (taken from the carb just above the closed throttle butterfly) is to reduce emissions at idle by raising combustion temps. Manifold vacuum applies full advance at idle, reducing combustion and engine temps.

Interesting... sounds like doing it this way would make it easier to tune as well, wouldn't it?

[SubGothius]

Interesting... sounds like doing it this way would make it easier to tune as well, wouldn't it?

Hm, not sure about that, but you would need to pinch/disconnect the vac advance hose in order to set your static base advance at idle properly.

[Chairwolf]

Just found this little gem for those looking for a kickdown linkage for auto trannies:

https://www.carburetion.com/Products...art=444.614.46

[solartony]

the Mikuni 32-35DIDTA carb has 5 mounting bolts to attach to the manifold ...


4 hexagon head bolts are easy to locate around the double barrels opening, in a rectangle configuration, ...




where's the fifth bolt ?

[kentleybeats]

I believe that is for the Weber carbs, not the Mikuni. Also, thanks for the 7 pages of info, all. Need to track down a vacuum leak on my Weber 32/36.

[SubGothius]

Webers only have 4 bolts, also in a rectangle pattern but different from the Mikuni pattern, hence the need for adapters.

Looking at the base of my old Mikuni I only see the 4 bolt holes, so perhaps any 5th bolt may depend on the model year or engine spec. I notice some adapters have a spur off the main rectangle, so that's prolly where the 5th bolt would be if present for your year/engine.