Friends, I need some help. I understand this is a tad unorthodox because the subject in question isn't a motorcycle, but instead a four wheeled counterpart.

1977 Corvette, 405, TH200R4.




I have limited skills when it comes to tuning and timing as it is and working with these two Mikuni PHH44 carbs has me stumped.

They have larger barrels and jets that I was told will support my engine and cam. Todd and Wolf Creek Racing helped on that if anyone's familiar with him.

The engine does start but is very cold blooded. It needs to warm up completely before it can be driven without bad stumbles and backfires. Once warm the idle drops and it's manors improve but I still feel something is wrong.

I dropped the car off with a local mechanic who's quite good at these things and did some timing work playing with the advance and weight. Not much to report for improvements. The mechanic made a statement that I did not agree with, "Not enough CFM from the two Mikuni carbs for my SBC."

I have read that a Mikuni like mine can have anywhere between 180 and 210 CFM per barrel. Since I have a total of four barrels I would think 720 to 840 CFM would be more than enough.

One of the biggest symptoms aside from the rough cold starts and odd idle speed, is that it will still stumble and act as though it wants to die if I goose the throttle at idle. My knowledge and experience tells me a stumble is indicative of not enough fuel when punched. My interpretation is that if these carbs were too small for my engine it would still react good but lack the power of a higher CFM setup.

I personally think my carbs can provide enough fuel and air to run the engine. Is that a true assumption?

What steps do I need to take to get this running a little better? I don't mind taking this to my local HD dealer and finding a good bike mechanic that has experience with these Mikunis. I have been asking car guys about this project and now I'm picking the brains of bike guys.

Thoughts?

Thanks.

 

Don't think we are going to be able to help. Mik 42s and 48s are the Miks of choice for modified Harley engines but they are very different animals from the PHH44. That's not to say that there isn't a forum member that has some experience with those cams. Good luck.

 

I can understand the uniqueness of running a setup like that but I think the headaches caused would out weigh that effect. Sorry, I have nothing useful to add but it "looks" cool.

 

Thanks for the compliments, and yeah I understand it's a long shot and may not garner much applicable assistance. I am willing to literally put this out there for any group of guys that might have an insight.

The 'WoW' factor is certainly there with this engine. So far the issues have not compromised the worth of doing this engine the way I did. In fact since it does run and drive fairly well I'm extremely happy.

It get the idea that there are just a new little stumbling blocks keeping me from enjoying the full potential and I may have to exercise patience in discovering and defeating them.

 

I looks like your electric choke(one side only) is set too lean. Accelerator pumps help eliminate the stalling tendencies, they can usually be adjusted. Once warm and still stumbling, go richer on idle circuit jets.
Mikuni should have tuning manual available for this carb type, it would really, prpbably online download. Once you have it running well enough to drive, take it to a tuner/dyno have AFR varified. Good luck M2C FWIW Fastrider124

 

Yeah you're right on track with a few things we've explored. I actually added the choke, scratch that, BUILT, then added, that electric choke. Before I wasn't using it and it was very difficult to start. It's a very cold blooded engine without those start circuits being used.

However, they lean the engine out so much that the header primaries glow cherry red and start melting things. So, I decided to use just one of them and even now I'm thinking I'll have to limit it to only half travel. Being electric, it's only on for roughly 8 minutes and half that is nearly closed.

The accelerator pumps were supposedly adjusted and I don't know if the jets are rich enough or not so that is on my list. Last time we tried to tune it we used a 3 gas probe to read the exhaust mixture and we couldn't get it to balance right. It was always lean at idle and rich above idle. If we richened it up it fouled the plugs. If we leaned it out it started heating.

This would be easier if I could find a Mikuni expert here locally but most mechanics are lost unless the carb has a Holley or Edelbrock stickered on it.

Keep the ideas coming. At this point I plan to look into the accelerators...which are working, and the size of the jets. Thanks everyone, for everything so far.