Jetting nightmare

[Greenstreak]

I just bought a 1972 CB750 and the jetting is messed up. The bike has pod filters, Dyna S ignition, and Triple A headers with an open megaphone. I am looking to get a jet kit that is close to making the bike run right. It spits and sputters but generally runs ok at full throttle. I am in Tucson Arizona and the elevation I ride is between 2,000 feet and 4,500 feet.
Any help with jetting this fine bike? Thanks in advance!

 

[dirtdigger]

No one can tell you what your bike needs for jetting. Too many factors contribute to that. Engine condition, tune up condition, how well the inside of the exhaust collector is designed, temperature, fuel quality, how you ride all affect the jetting. Before doing any jetting you really need to get your tuneup items up to factory spec as well as make sure your engine is mechanically sound...a wore out engine will never run good no matter how you jet it. Also sounds like you should make sure you dont have any plugged passages in the carbs and need to verify what your jetting is now as well as needle position and float level. There really are no jet kits for these old bikes. Go to Jetsrus for your jets as they are cheaper then anywhere I have found.

Do you have any experience in jetting? A lot more to it then just putting in different main jets. These round tops have screw in pilot jets as well as adjustable needles which you have to do some carb disassembly to change. Needle position is one of the most important ones and the most difficult to change. Because you dont know what you need I suggest going back to stock or one size larger on the main jet, stock needle level and see how it runs at different throttle openings and speeds then adjust from there depending on how it runs.

 

[amc49]

X2.............five identical bikes in a row and once good and well used every single one of them will need different jetting. A crap header that does not work well can actually force you to jet DOWN instead of up like normal.

You NEVER jet EVER until the basic engine tune is dead on, meaning a full tune up to as good as engine will get and beginning with a compression test to make sure all effort is not wasted. As well, go back to the OEM jetting for that model then go from there. You got parts that should need more jet but the altitude then takes it all back again, so back to stock jetting until you prove out a need for more. The particular spark plug used alone can determine jetting. The timing and point dwell have a whopping effect on it as well.

People always want the exact jet needed and that so shows how little they know.............

 

[Greenstreak]

Thanks for the help. I have done quite a bit of jetting 2 stroke MX bikes. They were fairly close to running right when I got them. This one is way off, I thought maybe I wouldn't have to spend a lot of time messing with it, but looks like I will. I will start on the complete tune up next week.

 

[amc49]

If you've played with expansion chambered bikes, 4 strokes do not need nearly the big jet difference an added chamber can require on a 2 stroke. Often only a couple sizes up in main and occasionally one needle clip change.

 

[dgreenbluet]

X2.............five identical bikes in a row and once good and well used every single one of them will need different jetting. A crap header that does not work well can actually force you to jet DOWN instead of up like normal.

You NEVER jet EVER until the basic engine tune is dead on, meaning a full tune up to as good as engine will get and beginning with a compression test to make sure all effort is not wasted. As well, go back to the OEM jetting for that model then go from there. You got parts that should need more jet but the altitude then takes it all back again, so back to stock jetting until you prove out a need for more. The particular spark plug used alone can determine jetting. The timing and point dwell have a whopping effect on it as well.

People always want the exact jet needed and that so shows how little they know.............

X2 on the advice to start with original spec jet recommendations!

 

[Wayfastwil]

carb jetting info

Did you have any luck? As one poster said make sure the bike's engine is in tune check the compression and correctly adjusting everything else. Don't forget the float bowls. Plug chop your engine I use #4 cylinder the plug will tell what is needed be sure you have the right plugs I start out with a new or nearly new set. I test a carb setup for leanness simply put a large piece of tape across all 4 of the pods this will let less air in so if your carbs are lean it'll run a little better plus the higher you go in elevation the leaner a carb setup gets. On your carbs do the needle's use spacers or have clip adjust? If you have clips good for you I'll bet you'll need to add 1 or maybe 2 spacers under the needles. With the pods you have to dead on with the tuning & jetting. You will lose some horsepower unless your bike is. The slow speed jet sizes start at around 38 thousands I like 41's my bike starts good & runs well there. I don't know if the slow speed jets are available anymore try. You may have to drill yours out do this by hand measure & measure again. Drilling carb jets has to be on the money. If you don't have a shop manual you need to get one. As far as jet size. Re: the slow speed jets if correctly jetted it should run good to about 4000 RPM then the carbs transition from the slow speed jets on the needles and into the main jets for the upper RPM remember all engines have a peak HP output meaning the redline is not where your engine is making the most power. With pods, updated ignition & header I'd start with 130 go up or down one size at a time. You may have to go to 150's Good luck

 

[dirtdigger]

All you have to do is use your choke while riding in the throttle opening you are trying to diagnose...slowly closing it will richen it up, if it picks up its lean if it bogs down more it is already too rich. DO NOT drill out jets. It isnt precise plus you can buy them at JetsRUs. A stock motor should never use more then a 125...130 at the absolute most with a bad exhaust setup. My big bore over 1000cc motors dont use more then 135 mains. I have a set of 140's and they have never been installed in a motor in 20 years. If it takes a huge jet something is wrong.

 

[Wayfastwil]

Everyone has a different way to diagnose problems my tape method has served me well I'll continue to use it. We used to drill jets back when because that's all we had. We even experimented with an epoxy type glue to fill & redrill jets before we used what precious jets we had there was no JetsRus store back then. BTW I don't think Mike at JetsRus has the slow speed jets they are very hard to come by but I can't speak for him. I have 140 mains in my 1978 750f with pods, electronic ignition system and a 4into1 aftermarket slipon. My 750f runs well no doubt you could do a much better job with the tune and of course your 1000cc engines will run circles around it. This why I seldom reply to anyone asking for help. Since we have become attorneys instead of guys on bikes trying to help other guys on bikes, I'll add a disclaimer If I ever post anything else please consider I've had brain injuries from motorcycle wrecks and ain't to smart anyway. Will in W Texas

 

[dirtdigger]

For everyones information Jets R Us has slow jets for roundtop and keyhole carbs, just checked again because I hadnt ordered from them in 6 months.