High I have a 75 cb 750 f and i'm new to the forum

[M Saunders]

High my name is Matt i have a 75 cb 750 f with a RC installed 836 kit that has bin in my family since it was new. My dads uncle was the original owner all the organelle paperwork on the bike was lost over the years. I would appreciate any info i could get on the rc 836 kits and could anybody tell me why RC unhooked the starer i was told to much compression but it seams to run good on 93 octane i would guess not much over 10.5:1 ratio but it does idle rough like it has a came lope.

 

[dirtdigger]

Was this a race bike? The only time RC got rid of the starter was when they also eliminated the heavy flywheel. This was only done or race bikes to save weight and reduce rotating weight in the engine. Compression ratio isn't an issue with the 750 starter normally as they work well into the 12:1 ratio. I would like to see copies of the original paper work from RC, not many people save them and then I could tell more of how the motor was built.

 

[M Saunders]

Was this a race bike? The only time RC got rid of the starter was when they also eliminated the heavy flywheel. This was only done or race bikes to save weight and reduce rotating weight in the engine. Compression ratio isn't an issue with the 750 starter normally as they work well into the 12:1 ratio. I would like to see copies of the original paper work from RC, not many people save them and then I could tell more of how the motor was built.

No as far as I now it wasn't a race bike the motor is mounted in a stock cb F frame. I don't have any of the RC paperwork i am going to ask family members that owned the bike in the past but i doubt i will have any luck. The only visual sins the motor might not be stock are allen bolt kit on the clutch case, KN air filters, black rinkle panted heads it doesn't have anything added externally on valve cover for cam support. I think it was probably built up as much as you could and still maintain a daily driver. It has sat for 16 years i added gas and kicked it 10 times and it started i have not dun any tuning on it sense i had it running when i was 23 wen i parked it last front tire was bald and didn't have the money to replace it. It runs good but i would like to get it to run the way it did after it left RC my Dad thinks it is i don't think we are getting everything out of it yet. I got a lot of little questions about tuning up the motor like should i run stock timing should i run a baffle in the muffler it is a old rc muffler with no baffle and factory 4 to 1 header. When my dad got the bike it had no baffle in the muffler i found a baffle for it but it seams to restrictive i loose power i might not have the right baffle it has a small hole just a little bigger than 1 in?

 

[dirtdigger]

Yes run stock ignition timing. I would be curious as to what cam is in it, it would determine what the valve clearance should be set at, the RC cams used a lot bigger clearance then most cams of the era. The 836 was a common hot street motor setup. There is no reason that the starter was disconnected unless the internal drive was eliminated. Is the starter still mounted on the motor? Or has it been eliminated completely? You could pull the alternator cover and see if the starter drive and flywheel are still present, if they are there is no reason you couldn't hook the starter back up. Get it good and warmed up and do a compression test on all 4, mmight give a good indication of compression ratio and cam. The cam support in the valve cover was only used in the earlier years at RC with very large cams and stiff valve springs. As the years went on and specific springs got developed for the performance cams for the 750 the support was no longer deemed necessary unless all out racing. Do you know if the motor was actually built at RC or was it built from RC parts but at home. RC sold more parts then he sold motors, many people just bought the parts and built there own.

 

[M Saunders]

Yes run stock ignition timing. I would be curious as to what cam is in it, it would determine what the valve clearance should be set at, the RC cams used a lot bigger clearance then most cams of the era. The 836 was a common hot street motor setup. There is no reason that the starter was disconnected unless the internal drive was eliminated. Is the starter still mounted on the motor? Or has it been eliminated completely? You could pull the alternator cover and see if the starter drive and flywheel are still present, if they are there is no reason you couldn't hook the starter back up. Get it good and warmed up and do a compression test on all 4, mmight give a good indication of compression ratio and cam. The cam support in the valve cover was only used in the earlier years at RC with very large cams and stiff valve springs. As the years went on and specific springs got developed for the performance cams for the 750 the support was no longer deemed necessary unless all out racing. Do you know if the motor was actually built at RC or was it built from RC parts but at home. RC sold more parts then he sold motors, many people just bought the parts and built there own.

This motor was built by RC in 75 I took the starter cover off today an discovered it does have a starter. I will pull the dino cover this winter when i am dun riding and see if the fly will has starter teeth. I also plan on pulling motor and valve cover i want to replace old valve seals and clean every thing up i will look for cam markings. I am also thinking about replacing head gasket it is not blown but there is a little dirt collected around it i would rather replace it before it blows out of ridding season. I also plan on going threw carbraters this winter and changing points to electronic ignition. This is the only Cb 750 sohc bike i have ever ridden or worked on it has sum unique characteristics it idles so harsh that you can hear the primary chain slapping but if you pull back on the throttle with a load or not it will not miss or hesitate and if you are on it at all about 5500 to 6000 it really wakes up like hitting the power band on two stroke. I have spent time in the past to try to get it to idle smother sync carbs to best of my ability and set timing to for factory specs i could never get it to idle any smother. Do you have any recommendations on things i should do to this motor ? I also do plan on taking your advice on checking compression on all 4 cylinders i need to get a compression gauge or adapter for mine that will fit into spark plug wells.

 

[dirtdigger]

If the starter is there then it was disconnected for some other reason as RC would competely remove starter and block the hole in the block when they did their starter deletes. Did you sync the carbs with vacuum gauges? Its the only way to get them spot on. Also idle needs to be in the 1000rpm range. Primary chain slap noise is common especially with a motor with some miles on it but generally excessive noise is caused by bad carb sync or idle that is too low. You have to split the cases to replace the chains if they are worn. The timing chain cam make a loud noise also if it hasn't been adjusted or if the adjuster wheels are worn badly.

 

[M Saunders]

If the starter is there then it was disconnected for some other reason as RC would competely remove starter and block the hole in the block when they did their starter deletes. Did you sync the carbs with vacuum gauges? Its the only way to get them spot on. Also idle needs to be in the 1000rpm range. Primary chain slap noise is common especially with a motor with some miles on it but generally excessive noise is caused by bad carb sync or idle that is too low. You have to split the cases to replace the chains if they are worn. The timing chain cam make a loud noise also if it hasn't been adjusted or if the adjuster wheels are worn badly.

It was around 16 yrs ago when i sync the carbs i did use 4 vacuum gauges made for sync carbs it was a set i borrowed from a fend i don't think it was a very good set. If i remember right i struggled with the vacuum being really erratic causing the gauges to bounce alot i will by a new set next spring and sync the carbs after i rebuild them. I have the idle set a little high 1400 rpm at 1000 it idles really rough and wont stay running cold at about 2000 rpm the motor runs real smooth and you dont hear any chain noise. I now it is the primary chain because it goes away when you pull in the clutch. I wouldn't think the primary is worn out the bike has under 12000 act miles but i did have to rebuild the clutch i used a a barnet clutch set made for dynoman the old clutch did not look worn out wen i pulled it but it slipped really bad it does not slip with new clutch. I did adjust the timing chain and don't seam to hear any noise from it. So do you think even with a aftermarket cam i should still get it to idle smooth at 1000 rpm and the carbs a probably the reason it idles rough?

 

[dirtdigger]

Without knowing what cam is in there it is hard to say how it would affect the idle. RC made a fairly mild cam for the smaller motors but he also made some very large cams. Only way to tell is to have a look at the cam and see what marking are on it. It will be marked RC ### on the end of the camshaft. Once you get a chance to take a look at the cam send some pictures of the numbers stamped and I can tell you what cam you have. Until you know what you have there is no way of saying if your very rough idle is a result from the cam or other things. The clutch basket can make a bad rattle noise if the rubber dampers inside are broke or hard...this noise will get less when the clutch is pulled in. Normally the chain noise will still be present with the clutch pulled in as they are still moving with crank even when the clutch is pulled. 12,000 miles is a fair amount of miles for an RC performance motor, the chains can very likely be worn out in 12,000 miles...they tend to stretch fast especially when you start adding power to the stock design.

 

[M Saunders]

I wanted thank you you for your help and after the riding season is over i will pull the valve cover and send you picks of the numbers on the cam. I am also reaching out to previous owners and will let you now if i find out any more info on how RC built the motor or if i can get paper work on it. Thanks again for all your help and i appropriate your knowledge and i plan on staying in touch.

 

[dirtdigger]

I love the old RC stuff and keep adding to my RC collection. I am in touch with RCjr who is rebuilding his dads top fuel bikes from the 70's to use as show and exhibition pieces. They are neat pieces and if you are interested there is a facebook site dedicated to the rebuilds. RC Legacy Motorsports. Any more questions about the RC stuff just let me know.