78 235 johnson carbs

[jy118lfd]

I am looking for info on 78 235 johnson carbs. Mine seem to run lean at lower rpms and I have cleaned them 3 times all orfices are clean but one idle jet is stuck in top carb and looks like someone screwed it up tring to take it out. I have a set of v4 140 carbs but I dont know if all the jets are the same. If I can get the jets and sizes I may be able to use these. The motor runs fine at any throttle above 1/8 and under load you can hear and feel the motor stop missing and the power comes on. It will run perfect all day long at higher rpms and throttle<br />Any suggestions would be greatly appreciated

 

[Dhadley]

Re: 78 235 johnson carbs

Its not unusual to have to re-jet carbs that old. When they were manufactured they were calibrated for fuel available at that time. We no longer have that fuel. With all the oxygen bearing compounds in the fuel today, some older motors tend to run leaner.

 

[jy118lfd]

Re: 78 235 johnson carbs

I understand what you are saying but I dont know how to rejet or where to start. None of the books talk about rejetting a fixed jet carb. If anyone out there knows about these carbs let me know

 

[Dhadley]

Re: 78 235 johnson carbs

Unless you know someone with some extra jets it could be a bit expensive. Basically if its lean at idle you just re-jet the idle circuit. If you are dealing with air bleed jets in the idle circuit you have to go smaller to go richer as the meter air not fuel.<br /><br />Just make sure thats what the problem is before throwing jets at it. Make sure youre getting good spark and the carbs are clean. Bottom line -- re-jetting is pretty much a last resort.

 

[DHPMARINE]

Re: 78 235 johnson carbs

Jon,before you even try to rejet check the throttle bore.That is the inside diameter of the hole thru the carb.I would imagine there is a decent difference between the two,and you'll never get enough flow thru the carbs for it to work.Think of an engine as a pump.Air in,air out.If you run the 235 lean,you will need pistons and cylinders.<br />DHP

 

[jy118lfd]

Re: 78 235 johnson carbs

The throttle bores are the same between the carbs I have measured them and the bore is cast on the main body. I am going to take the carbs off and make sure the original carbs do not have any plugged orfices but I have done this twice and had no luck. Maybe since I have run it something found its way in the idle circut. thanks so much for all the help