allright guys I'm going to have to call on all our rotary knowledge to solve this problem before I take a sledgehammer to my Mikuni PHH44. Here's my issue:

The car runs like a raped ape when I'm accelerating, never experienced the dreaded "Mikuni Bog" once (thank God!!), but once I get to a good cruising speed and try to maintain it she begins to buck, sputter, miss. If i tip the throttle in a little more it clears up, but as soon as I roll back off the throttle it starts again.

Now here's the real kicker, thinking that perhaps this could be ignition related I've rewired the entire system and put in new plug wires and new plugs. Also when I pulled the plugs I noticed that the front housings plugs looked very rich (black, sooty) but the rears plugs looked lean (light brown, flecks of white) ????? Totally lost on an explanation for that one. Now none of these problems existed prior with my stock nikki set-up.

Now for my Set-Up:
81 12a with small port on secondaries ONLY
Holley Red Pump
Profuel Regulator set at 4.5 PSI (verified by inline F.P. Gauge)
8mm plug wires
2xMSD Blaster Coils
RB Header
Full 2.5" Exhaust w/ Borla XR-1 Presilence and Magnaflow muffler
Mikuni PHH44 on Racing Beat 2 Piece mani and plastic carb spacers

Jetting is as follows:

Pilot:67.5 (but have tried 62.5 and 70's and no improvement)
Jet Block:OA
Main Fuel:190 (but also tried 200, and 210's)
Air Corrector:240 (but also tried 220)
Pump Jet: 90

I appologize for the length of the post but I wanted to make sure I listed all the variables so everyone has all the pieces to the puzzle. I'm just not ready to give up on this carb yet so help me out guys!!!! Thanks again.

Ken

 

Looks like too much air. Recommend you try 190Fuel, 200 air.

 

I'm going to give that a try doc. But I thought the main circuit is only used after 20% throttle. The problem I'm having is under 20% throttle.

 

bump!! help please!!

 

check the engine compression.

 

engine compression should be good...it ran perfectly fine with the nikki... I don't know why changing a carb would make me lose compression??

 

Since the front is running rich, it might me time to clean the carb, that and perhaps turn down the idle mix in the front a bit.

 

Sorry, I meant to say clean the channels and jets for blockage. "Compression" was my reply to an e-mail but posted it here in error.

 

I've got a Dellorto but they are similiar. Since it gets up to crusing but then sputters under the cruise condition unless you pump the accelerator, then I would think its a clogged passage in the carb, probably on the rear rotor side of the carb since your seeing a lean condition there. My reasoning is that when cruising you are not using the idle cicuit anymore and the carb has progressed to the main fuel circuit. By pressing the accelerator you are basically causing the accelerator pump to shoot out some gas into the venturis momentarily enriching your mixture.

First, I would try cleaning it by just spraying carb cleaner on and in the carb while the car is running. Soak it good and maybe run some seafoam in a tank of gas. If its a minor dirt issue in the carb this may knock it out and solve the issue. Don't adjust or change any settings if it was running fine before this and it just started happening. Also change your fuel filter(s). You have more than one right While your at it change the air filter as well.

Next I would pull the carb and rebuild it. Clean it good and replace all the soft parts. Also for the mixture difference make sure the idle mixture adjusting screws are set identically, meaning the same number of turns out for your idle. I doubt this is your problem since it happens on the main circuit. Warning messing with these will mean retuning to get rid of any bog you may get as the carb progrresses from the idle circuit to the main circuit. One other thing is to check and make sure the idle mixture jets are not bent or clogged.

 

well I do have to 2 fuel filters installed one pre-pump and one post regulator pre-carb. both are new and both appear to not be clogged. as for airfilter i'm waiting on the racing beat element to get here but in the mean time I've been using a couple pantyhose over the velocity stacks to keep out as much dirt as possible. Not my idea set-up but RB has been a little slow shipping my filter.

That being said I was kinda thinking the same thing all you guys are posting..blocked jet/passage. the problem is I don't have a copy of the mikuni manual to help wth a tear down and rebuild, otherwise i would have done it before installing the carb. I mean is it a simple process, will i need any specialized tools or techniques? If it's fairly straightforward I'm pretty mechanically adept I figured I could just tear it apart and bag all the pieces in ziplocs and mark them individually. Same way I tear apart engines and what not. Just don't want to get half way into the rebuild and not be able to get it back together since it is my daily driver. Also what's the best way to clean the passages?...Compressed air with a little solvent? I mean I'm willing to put in the work to make this thing run right just might need a little more guidance guys. Thanks a million.

 

I think your filter let some gunk in. I wouldn't clean the carb up and fix this until you get a real air filter. I bet the pantyhose get all stretched out and let crap into the carb and that may have f$%#ed things up for you.

That said I did a search and came up with these two items. Hopefully they help.

http://www.toysport.com/webpages/Pro...rs/Mikuni2.jpg

https://www.rx7club.com/1st-generation-specific-1979-1985-18/so-i-got-my-mikuni-manual-413156/

 

yeah filter should be here tommorrow or so, i'm thinking it just got a lil gunked up from sitting around for awhile. what would be the best way to go about cleaning the passages/jets?

 

Like I said earlier, grab some carb cleaner, start it up, hold the throttle open with your hand and spray into the carb but keep the revs up. Do that a few times and then see if it helps. Next would be to disconnect the fuel inlet and spray a bunch of cleaner in there, put the fuel line back on and start it up and keep it running until it stops stumbling from the cleaner. If your careful you could pull the top off and hose down the float chamber and any jets or holes from there. If none of this helps you might as well pull it off and get with the rebuild.

So how long did it sit since it last ran? Maybe you have varnish in there, then a rebuild is almost unavoidable.

When I rebuild I used a carb cleaner aerosol (gunk) with a small tube on the spray head an I stick it down in all the holes. While its still wet I grab my air sprayer tip on the compressor and aim it into the opening and try to blow it all out. Sometimes I get a container, usually ceramic, and fill it with carb cleaner and soak the removable parts that I can and blow them out too. Make sure you use a carb cleaner, some solvent may attack the alumnium and ruin the carb.

 

ok..sounds fairly straightforward. I don't have access to compressor for air but i was thinking that maybe i could use the canned computer duster stuff. the carb sat for about 4 months before i installed it but possibly longer before i bought it. I'm going to try the carb cleaner route, rather than tear i all apart. I've been able to acces the fuel bowl fairly easily and maintain the integrity of the gasket I'm thinking maybe i can get in there with some carb cleaner and pull the jets and block and shoot some down in there. If that doesn't get it I guess it'll be time for the rebuild. Correct me if I'm wrong but that should get to those passages pretty well, right?? I would just rather not go through all the trouble of pulling the venturi's/throttle plates/acell pump yada..yada..yada.

 

Venturies, jets, airhorns need to come out to properly clean the passages. I use brake cleaner along carb cleaner plus compressed air. No compressor? no problem. Bring the carb with you on your next gas fill-up. Be sure to clean the jets also when you put them back. Dont use the venturies. It will make it run better.

 

If it is bucking or sputtering while crusing than that is an indacation of a too rich or too lean mixture. But you have already established that. While cleaning fix it right the first time, new gasket set and I would not suggest compressed air it could possibly damage something, rather take a wire brush and pull out one wire with a pair of plyers. It is sprung steel and will not break but strong enough to breakup any dirt or build up. If you are not sure about dissasembly and reassembly take pictures with a digital camera. Good luck and take your time.

 

The piece of wire is too clean any small passages, holes or ports.

 

thanks for the heads up on the wire brush... I'm going to pul her off tomorrow morning and see if i can't get her runnin a little better. I'll let everyone know the results. thanks again guys.

 

Mikunis are very simple to tear down and go through. I went through my 1st one with no instructions and no rebuild kit. It had sat in a junkyard for years and was full of varnished fuel. Installed it and it fired right up.

Carb cleaner, brake cleaner and compressed air will take care of most of the cleaning. When you pull your emulsion tubes, remove the air and fuel jets at the top and bottom of them for cleaning. Remove and clean the pilot jets too, as this may be where your clog is.

 

thanks trochoid some how i caalways count on you to give me that extra bit of confidence to dive on into whatever problem I have. Going to the parts store to buy a couple cans of cleaner and start tearin her down. Wish me luck have to get it done in about 6 hours to go to work

 

ok guys here's a litle update for everyone.... tore the carb apart yesterday.... and three cans of carb cleaner later, my car is running like a champ!!!!!!!! Idle is still a little rough but I'm thinking the pilots are just a little too big, I know I heard trochoid say he's running 52.5s, i believe, and I'm still at 62.5s. So I'll check that out once I get my Wideband. I also took wacky's advice and removed the chokes to make the carb a true 44 and what a difference I thought I'd lose alot of my low and mid range, but I was wrong, feels healthier through the whole powerband, thanks Mel! so here's the final set-up from using the butt dyno.

Pilot:62.5
Jet Block:OA
Main Air:220(made a huge difference from the 240, thinking about going to 200)
Main Fuel:200
Pump:90
No Chokes
5 PSI on fuel

So everybody give that a once over and give me any recomendations for jetting if we think there's anymore power to be made.

Thanks again guys for all the help!!!!!!!

 

Wasn't that hard was it. lol. My pilots are probably too small, I have a larger pair I should put in. I may try taking the chokes out, I'm sure the 1/2 bp probably needs the flow.

 

Glad it worked out for you BUT I dont think that will help Mr. T. He needs the almightly 51.5 IDA the least. I have no experience with the Gene Berg special but I heard good things about it.

 

Recommend 57.5 on the pilot jet. 200 on the air jet. Should get you real close.

 

Yeah, I know I need a bigger one, but I can't see buying a high dollar carb when the engine is coming out for the TII swap. I'll let the next owner worry about carburation. Unless someone has a 48-51 mm carb they want to loan out or part with cheap.