poor running mid range speed

[tiupc]

I have a 75 Ski Sporter with a 2011 crate 350 in it. Runs great at WOT ( 4800 rpm) and at idle. When slowing down below about 3800 rpm, the motor seems to have a miss in it. Opening the throttle, it will speed up to 4800 rpm again and seem to run fine. Motor sounds like it is missing from about 2600 - 3800 rpm. I changed from Corsa Thru hull to Straight thru hull exhaust. This was when I noticed the miss in the engine. I have changed the plugs, wires, Distributor cap, rotor and the fuel filter. Motor has less than 30 hours on it.

Any suggestions

Thanks
Trevor

[Tidbart]

Could be a carb issue. Is this carbed? If so, what type?

[tiupc]

Holley 4 barrel I think a 4160. I will check tonight as the boat is at the cottage. I should also be able to tell if manual or vacuum secondaries ( there is a vacuum outlet on back of carb, choke side but it is blocked off. Also, choke is not powered, never has been.

Thanks
Trevor

[biggiefl]

A 4160 is Holley's vac carb...600cfm but they also have a 750 under the same #. Something is weird obviously. Why did you disconnect the exhaust? Was it running fine before? If so you probably knocked off a vac line or ironically something went awry. I had a 4160 on my old Nova and was constantly having problems with it. Vowed never to own one again but it was just that particular carb that had major float problems.

[MOP]

By getting rid of the Corsa which are mufflers you may have a back pressure issue, you may need to do some jetting as you can be running lean now. Best advice on this issue will come from Buzilla. It would be a good idea to pull a few spark plugs, post a couple pf shots up here for us to take a look at.

Phil

[tiupc]

I looked at the carb horn and the only numbers are 6057-5 and underneath 3186. The secondaries are mechanical but when I open the throttle on the carb, the secondaries do not open. The linkage does not pull far enough. the motor runs up to 4800 - 5000 with a 280 drive Solas 21 pitch Titan. The carb does have a vacuum fitting on the choke side but this was blocked from when I got the boat and there are no lines or hoses any where else on the carb. I did notice a small tube coming off the mech fuel pump but I don't see any where for this to join to It is about 2 ft long. The Corsa's don't appear to be mufflers as the sound is the same . I changed to straight tip to match original more closely. I will pull some plugs and post pictures. I also will put the Corsa's back on to see if this makes a difference. If anyone can tell me about the Holley Carb as I can't find info on it. I have a feeling this isn't a marine carb. Any info/help is greatly appreciated

Trevor