engine and drivetrain Mikuni Dual Carbs

[bmwfan]

I recently aquired a '72 2002 that has some know and unknown modications. dual Makuni carbs, headers, not sure on cam, but idles like it has been modified, rear end gearing is tight. 

 

It did not run " right " when I bought it. Bad engine shake, which was fixed by plugging 3 open vac ports on the carbs and replacing vac tubes. Tune up, changed filthy gas filters, in-line and in Mikuni feed line filters cleaned points, set dwell to 60, multiple timeing adjustments. Currently set at " Z " mark on harmonic balancer at 1500rpm.

 

My issue is that through shifting gears the car will either bogg down under light acceleration but if I accelerate hard it will fly. Head back home , make pilot screw adjustments using tach, re-set timeing, head back out , now  runs smooth under liight acceleration but boggs down under hard acceleration.

 

Also, at what I think is the correct timeing, dwell, carb adjustments, I  can not get the idle under 1300 RPM. The idle screws are backed out. The linkage is not binding, throttle closed.

 

I have received some help already for members of this forum. And used their info. and info on Wolfcreekraceing site, but now at a dead end. I would apprecate any additional information.

 

Thanks, Walt

 

[TobyB]

My experience has been with Webers, so take it with a grain of salt-

 

first, have you set your float level?  That will cause all sorts of problems if it's not in the ballpark- or you have a sinker.

 

Second, have you synchronized the carbs? There are several tools out there, but the synchrometer is the one I like better...

 

You say set timing by harmonic balancer- do you mean flywheel?  the ball bearing's the usual standard, at something between 1500 and 2k.

But if the distributor's a mess, sometimes you get better results by deviating from that....

 

How is fuel getting to the carbs?  Too much pressure can overpower the needle valves, and that can also cause tuning trouble.

 

If the idle stop screws are backed out, and the linkage is not holding the throttles open, the car should stop dead.

Unless there are holes drilled in the throttle plates.

If you have unperforated throttles, look carefully at everything- SOMETHING is holding at least 2 open-

and it may be the other two in the other carb!  Or it may be a jammed shaft, an offset plate... you just have

to dig into it and see what it is.

Unless there's an idle air bypass on the Mikuni (late Webers have them) and then you need to tune THOSE to

get your idle down.  But I bet they don't have bypasses- that was a recent innovation.

 

Is your choke system working?  And disengaging?  On the Weber, the 'cold start circuit' is a little carb in and of itself,

and if it's engaged, it will run the car- and run it very rich.  If it's engaged, it'll be running the motor rich at low speed...

 

random thoughts...

 

t

[andyleonard]

TobyB's got most of it.

 

You state the idle stop screws are backed out, but is the main pedal linkage still holding something open? Or the connection between the carbs? If not, you probably still have a vacuum leak....base gaskets, vacuum hose to booster, etc. Spray some carb cleaner around and listen for the idle to go up.

 

The smooth idle and stumble on hard acceleration would indicate - given how out-of-whack this setup was when you got it - either non-functional accelerator pumps or seriously too-large venturis. Check the acc pump shot first.

[MarkDavis]

This is how I had my Mikuni's jetted. 

 

      Jet                                               40 mm Mikuni’s        

 

Pilot nozzle                 45                           

Accelerator jet           120                         

Main fuel jet             135                         

Main air jet                290                                                  

Jet block                     oa                            

Pilot jet                       57.5                         

Small venturi               10                           

Large venturi                 36       

 

Mark                

[bmwfan]

Thanks for all of the advice and jet info.

I reset timing to Z mark on front pulley , dwell at 60, re-adjust pilot jets. Took out for a run, seems to be much better. Strong performance with aggressive gas peddle and acceptable low end power. Still with a bit of bogg down with a light peddle. Thinking it is now just a fuel flow issue. Had already replaced see through fuel filter, had visible debris , removed both in carb inlet filters, had a massive amount of debris, sprayed off with carb cleaner, but still has caked on stuff that won't come off.

Ordering new in line filters from wolf creek, and while waiting on them will pull the tank sender to clean screen and inspect inside of tank. My do a por 15 seal.

If this all fails will do a carb rebuild. Or send them to wolfcreek for them to rebuild depending on their price. If I do the rebuild I will see how my jets compare to yours Mark.

Also Mark, for what type of driving/engine build is your jetting for ?

Thanks , Walt

[Harv]

Sounds to me like your timing is too far advanced. You said the idle screws are out so I'm guessing that the idle is too high and the setting is as low as you can make it. If the problem isn't something else the hesitation could be too lean or plugged idle jet (or whatever Mikuni calls it).

John

[Emyers]

I recently aquired a '72 2002 that has some know and unknown modications. dual Makuni carbs, headers, not sure on cam, but idles like it has been modified, rear end gearing is tight. 

 

It did not run " right " when I bought it. Bad engine shake, which was fixed by plugging 3 open vac ports on the carbs and replacing vac tubes. Tune up, changed filthy gas filters, in-line and in Mikuni feed line filters cleaned points, set dwell to 60, multiple timeing adjustments. Currently set at " Z " mark on harmonic balancer at 1500rpm.

 

My issue is that through shifting gears the car will either bogg down under light acceleration but if I accelerate hard it will fly. Head back home , make pilot screw adjustments using tach, re-set timeing, head back out , now  runs smooth under liight acceleration but boggs down under hard acceleration.

 

Also, at what I think is the correct timeing, dwell, carb adjustments, I  can not get the idle under 1300 RPM. The idle screws are backed out. The linkage is not binding, throttle closed.

 

I have received some help already for members of this forum. And used their info. and info on Wolfcreekraceing site, but now at a dead end. I would apprecate any additional information.

 

Thanks, Walt

Are you running 40's or 44's?

Earl

74 02Lux w/Mikuni 44PHH

02 M Roadster

72 Volvo 1800ES

[bmwfan]

Thanks for your advice. I am going to back the timing off a bit. But when I have done this before the idle does slow but engine runs rough. I agree the carb/jets may be clogged and causing the idle problem when timing retarded. Rebuild of carbs in the near future I think.

Don't know if they are 40 or 44's. I' ll find out, as the size is supposed to be stamped on the back of throttle plates, when rebuild starts. As far as I can tell on wolfcreeks site, rebuild kit is same for both sizes.

walt

[MarkDavis]

Walt

 

The engine

 

292 cam

9.5 to 1 compression

Stahl Header

2.25 inch exhaust

MSD and Hot Coil

 

I used the car mostly for week end fun and made it to the track once.

 

Mark

 

 

[bmwfan]

Removed all of the weird vac hoses and set timming and dwell10-15 times. It is running strong under hard acceleration, still a bit of a lug under normal street driving. I have what I think is a weak electric fuel pump, fuel barely flows into the clear filter. I have a new airtec ( sp ) pump waiting to install. I need to make some sort of adapter to attach mounting bracket to before I can place it. Looking at body cross rail that runs in front of the gas tank.

 

I hope that the new pump will make a big improvement.

 

How bad of an idea would it be to mount it inside the trunk ?

[autobon7]

I know this is old but curious if you got this worked out.

[bmwfan]

Actually, I did. Mad sure no vac ports open, replaced intake manifold gaskets. Checked float levels ,they were right on, opened carbs and removed whatever would come out and sprayed all jets/ ports ECT... I did not remove accelerator pumps as I did not have replacement parts for them, just in case I tore the diaphragm .

New electric fuel pump and a decent fuel pressure regulator. Set the timeing to the ball on the flywheel . I think my setting was off using the front pulley . Then using an rpm meter adjusted the air screws over and over again. Very slight adjustments, 1/16 to 1/8th turns back and forth. A test run every now and then. And damm I finally got it going awesome. The setting are very fine, I drove a few times with the air filters off, no popping, and strong acceleration through all rpm's. But when I put the filters back on it started popping again, no loss of power noticed though. So a slight air screw adjustment and back to no popping.

However, I do keep a screwdriver , as well as a few other tool in the back, for as needed basis.

So, what do I think fixed this? A little bit of each of the above. But mainly dirty carbs and timeing not set right. Actually on the timeing , as stated I set it to the ball/ flywheel mark, then made very slight adjustments by hand and ear to get the idle right, the set the idle adj screws to the rpm that I wanted.

If you have Mikuni's , you need to look at wolfcreekraceing's web page.

[ptbadboy]

Great work-up. I have Miks also. 44's. When you set the timing, did you split the ball, and at what RPM?

 

don

[bmwfan]

I split the ball from the drivers side. Rpm , I think, was 1500. I used the factory recommend rpm rec. the idle was a tad rough at the set split ball setting so I backed the timeing off just a little and idle smoothed out.

One other thing, when I removed the air filter mounting plate from the carbs which was a solid plate except for the intake I noticed a open orfice about the size of a quarter. With the float cover off I could see that it communicated with the float bowel. Not sure of it's function, but may be some type of air pressure regulator . So I cut out matching holes in the mounting plates.

It pops some with cold start up. But none when warm, pulls strong at all rpm's . Very happy with it. Not sue of my MPG, but it does suck the gas up. But the gas to fun ratio is worth it.

[Emyers]

Those holes above the main venturis on the face of the carb are intakes for the cold start devices.

 

Earl

1974 2002 Lux w/Mikuni 44

2002 M Roadster

1972 Volvo 1800ES