• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

Need Help with fuel delivery

mgutows1

1972 CB750 SOHC
Messages
10
Reaction score
0
Points
1
Location
Kent, Ohio
Ok so I am new to this forum and I have been searching around to see if I can find some help with my bike. I recently purchased a 1972 Honda Cb750 SOHC for $600. Lots of customs and mods. Here are the details that I am aware of:
-Accel Super Coil 140403 coil kit (NGK D8es plugs)
-Four straight drag pipes.
-Pod air filters.
-657B AMH carbs.
-new boots from carbs to engine.
I am new to building bikes and I need some help.
I just took the carbs apart and cleaned them. The first and second carbs had a little gunk in them but other than that they were not too bad. The bike will run and idle fine but whenever I take it out and ride, the bike doesn't run well at all. It keeps cutting out and kicking randomly as well as a few backfires and popping. I don't know whether this is because of fuel delivery or because of the pods. I have read that the pod air filters are hard to get to work on these motorcycles. I have been checking for vacuum leaks and I do not believe that there are any. The boots are all new and sealed properly. I do not know if the previous owner re-jetted the carbs when he put the straight pipes and pod air filters on. Is there a way I can check the jet sizes that are currently in the carbs? What jet sizes would be recommended for the set up that I have? I am looking for any and all help. Like I said I am new to the cb750 and I want to learn. I am in college and would like to make this a permanent hobby of mine. I know that there are already threads on this stuff but I have not found any on bikes that are specifically like mine. please bare with me I am new to this forum.
PLEASE HELP!!
Thanks,
Matt
 
Last edited:
Hi Matt, The reason I,m reponding is that I own the same Yr. and pretty much the same setup mac drag pipes and pod air filters, I am no expert on these by no means but some main jets are marked on the bottom as far as size, Theres a tread on here somewhere about pods and jets. I do not why but Mine runs really good with 135,s most Guys run 125,s Start with a good synchronization of the carbs first. Then go from there. You will probally get more reponces from Members that know far more than Me on the subject. Good luck on Your 72.
 
Thanks, Dan.
I am going to purchase a few different sizes and try them out. Did you change the pilot/slow jets as well when you moved up sizes in the main jets? My bike is firing right up and idling just fine but I don't know if changing the main jets will effect the outcome of the pilot/slow jets.
Thanks,
Matt
 
You need to start with the basics, do all the routine maintenance....check and adjust valves, check compression, check plug gap, check and adjust point gap and if they are pitted or burned replace the points and the condensors. Check to see if ignition timing is good and make sure the advance is working correct. Then do the carb sync.

Jet sizes are stamped on the jets so remove them and see what number is on them. Drag pipes are really inefficient at low and mid rpm and do take more fuel then a good 4 into 2 or 4 into 1 exhaust. 125 to 135 is a general jet range for drag pipes and pods and you will have to play around with the needle position. But every bike wants different jetting, absolutley to many things influence jetting to say for sure what a particular bike will need for jetting. Engine condition, fuel quality, altitude, temperature, riding style exhaust system, any engine performance parts will all contribute to what jetting is needed. There are many threads on here about the topic as it seems it comes up about every month. You really have to just play around with the jetting until you find what your bike needs.
 
I would listen to Diggers advice, He,s helped Me before and seems to know all aspects of these machines. I,m still running a 40 slow jet.
 
DD is on the money on this one. Check carbs last. Get the other stuff up to par first.
 
Thanks for all of the help so far! So here is an update: I checked compression and plug gaps which are both fine. I pulled the cover off today and checked the points. the 2&3 point looks a little burned up so I just purchased a points and condensor kit. I am going to install that as soon as it comes in. I will keep you all posted on what happens when I install the new points and condensors. Thanks again to everyone who replied!
Matt
 
Ok so I am confused as to what these random ports are (2 on carbs/ 1 on cylinder head cover). I have been looking in the maintenance manual and cannot seem to find what they are for. The didn't have any hoses attached when I purchased the bike so I am not sure if they are vacuum hoses or what. I have posted an album to my profile called "random ports" because for some reason it would not let me post them to this file. Any help would be appreciated.
Thanks!
 
The one on the valve cover is a vent for the engine crankcase, most of the time the tube is run to the back of the motor so oil vapor/mist doesn't go all over the back of the motor. Or you can find small clamp on filters to put on there. You need something so random junk doest fall into your engine. The ones n the carbs are bowl vents for the carbs and again normally tubes are just run to the back of the motor.
 
Thanks Dirtdigger for all of the help! Here is an update on my 750: I replaced the main jets with #135 jets and i also replaced the points and condensors as well as changed the gap. The bike is now running like it is brand new! It is responsive and pulling like crazy! Unbelievable the difference with the new jets and points. I ordered new spark plugs and the manual stated that I need NGK D8es plugs so those are the ones that I bought. I went to put them in the bike today and they are too large. The threading is too large to fit. Can anybody help me figure out what NGK plugs I need?
Thanks everyone who responded, especially dirtdigger you are a lifesaver!
Matt
 
d8es or d8ea are same plug and are the correct ones for the 750. What are the plugs in the motor? Unless someone has had some machine work done to the head the d8es or ea are the plug that all the 750's used. Possible you new ones are miss packaged or miss stamped.
 
I pulled the plugs out of the engine and they are marked as NGK Iridiums DR8EIX. photo-10.jpg Here is a pic of the one from the engine (top) and the new B8es (bottom).
 
I ordered D8es and they sent me B8es. I did not even realize it until now. Sorry for the misunderstanding and thanks for the replies
 
Update:
I've been riding the bike a little bit here and there just to try and get an idea of what else it needs. So far I am thinking I either need to bleed the front brakes or replace the pads; it is making a squeaking noise and slows the bike down when not accelerating almost like there is too much pressure on them when not applying the brake. Also, I took the cover off of the bike this morning and there was a small puddle of oil/gas (dark color almost like oil but smelled like gasoline) directly behind the left side exhaust pipes. I am not sure what the reason for this is. Could it possibly be because the new main jets (135) I put in are too rich or a different reason? Any help would be appreciated and Thank you again to everyone who has been helping out! I love this forum!
Matt
 
About the brakes, have you took the caliper apart and cleaned the piston etc? It sounds like the pads aren't backing off correctly, this sounds like the result of a semi seized caliper. Pretty simple job to pull the two screws out of the mount, pump the lever and eject the piston. Clean the whole mess, re-assemble and bleed.
Bleeding alone won't really do much to get the pads to back off. Usually bleed issues are identified when the brake fades not from staying clamped.
 
Back
Top