Over the past week I've experienced an intermittent acceleration problem. Engine is a 2003 TC88b with carb, basically stock with the exception of hi-flow air cleaner and V&H Big Radius 2-2 exhaust. It's been running fine since I purchased last May, and I've put about 10,000 miles on. Symptoms are a step change, where suddenly engine seems like it has lost at least 70% of its power. No other symptoms - starts, runs fine, no back-firing or missing - nothing except extremely poor acceleration and power. I pulled the plugs yesterday and they were a bit black but not too bad. Replaced them anyway, gapped at .040. Air cleaner was cleaned and recharged about 2 months ago. Dropped the carb bowl and nothing floating around, looked clean. Today I put in some Startron cleaner (about 1/2 oz and STP octane booster. Left the house and it was running perfect for about 45 minutes so I thought it was fixed. Then suddenly - like someone flicked a switch - back to extremely poor acceleration. Took into my indy and he could not find anything obvious - it acted up for him when he drove it - both with and without the air cleaner on - so I think we've eliminated that.

My thoughts are:

Carb problem? Maybe some particulate that is sometimes clogging jets or passages

Bad gas? Maybe this will fix itself in time - maybe I should drain the tank to expedite

Intermittent MAP? (manifold absolute pressure sensor) - this and crank position are the only 2 sensors on the carb'd 88 as far as I know, and there's not much that can go wrong with the crank position sensor - inductive pickup. Both control ignition timing

I'm thinking the next time it is performing well I should pull over and unplug the MAP sensor and see if the behavior is similar to what I experience when it's acting up. If so I'd go ahead and replace it.

Don't want to take it to a dealer (over an hour away and you all know the rest) until I've exhausted possibilities I could take care of myself. My indy is a small one man shop and doesn't really have any suggestions on what to try next short of changing parts. He does not think it's a carb problem - says that would probably not be intermittent.

Any hints / tips on what this might be would be most appreciated!

Sorry this is so long-winded but wanted to provide as much info as possible.

 

Gap plugs at 0.035 and see.
Then maybe the coil.

But since the bike was built gas has changed. Hesitate to say gone to Sh++ but changed for sure.
and the MAP could do this but seems as if this would show in several places


connections, I bet, are a good place to check.

 

Found it. Small tear in in slide diaphragm. I'm surprised it was intermittent - but it was very close to the edge - just inside the seal. Performance is better than it has been since I bought it, so I assume that it probably had at least a pinhole all along. Still have some playing to do as the mixture screw wants to be almost closed, so may either remove the single washer under the needle or go down one on the idle jet (presently 45).