Engine start issues

[Primetime63]

Hello all, So I have this 78 Cb750k that I bought off an old man a few months ago and have found out that it doesnt like to start! When I test rode it at his house I noticed the engine was already warm. He messed with the idle srew on the carb and choke to warm it up..After I took it home it sat for a few days and on my 1st ride I burned the starter cylinoid up trying to get it to start!So I replaced it and then would drain my battery trying to get it to start! I know these old bikes with carbs and points can be tricky, so I checked the points and they seem to be gapped right, checked the valve gaps as well.. Now when I want to ride it I have to jump it off my car battery and it still takes a while! I also have to mess with that screw until I hear it want to start and then mess with the choke. Once it warms up it seems to run ok, but I have to constanty mess with the idle screw until the rpms get around 1000 or so..It will then start the rest of the day and even kick start, but after that its back to hell! Could someone tell me what else I should check? Im guessing carb rebuild? I did pull it and spray it down with carb cleaner when I replaced the air filter, but never synced them. Also once it starts it will rev real high and I have to push the choke down some..Im new to bikes as this is my 1st..Any help would be appriciated.

 

[Primetime63]

Oh, another thing..My buddy checked my battery while it was running and said its not charging it and that I probably burned up my rectifier by jumping it!! Any easy way to check this?

 

[Marty]

I would pull the carbs and go thru them to see what their condition is first. May need new kits in them to get rid of any vacuum leaks, set up floats, clean all passages ect. This way you know what you have. You also can check your wires from the engine case to see if you get continuity for your charging circuit. I hope you didn't overheat your starter too trying to start the engine. Good luck!

 

[Primetime63]

Thanks, I was afraid I might burn something up! Luckily the starter still turns..So this most likely is just a carb issue? I've never worked on a carb..Should I order a rebuild kit? Aside from just carb cleaner spray anything I may need? I only paid 900 for bike so I can't complain as it does run nice once it starts. ..also read somewhere that if I unhook battery after its running and it dies I could have bad alt.is this true?

 

[Marty]

If I were you I would look for a new or used Chilton's manual for the CB 750 and get it. There is so much good stuff in there for identifying symptom's and the repair process for your bike. I have one I use myself. There is a section on rebuilding carbs that if you follow it you will have no trouble. Plus you will learn how to work on many other things you may have to address. Problems can be accumulative. Your timing may be off, the timing chain may be needing the slack taken out of it. The valves may need adjustment on rockers. You just have to go thru things to know where you stand to narrow down the cause of the main problems. I highly recommend getting some type of repair manual as it will save you a lot of time, money and trouble.

 

[dirtdigger]

Watch the Chiltons manuals...they revise them often...I have seen wrong torque specs in a couple of different versions for the 750. They do have some usefull information but for the spec and actual procedures nothing beats factory Honda. But any manual is better then none. Start from scratch, check, clean or replace the points and condensers. They adjust timing and dwell. Replace plugs. Check compression. adjust timing chain. Recheck valve clearances. Pull all the jets...mains, emulsion tubes and idle jets and blow through them to insure they are clean. Then do a carb sync and reset idle screws to base spec. You could be lean causing hard cold starting but with the choke on these carbs dump a good amount of fuel. I am leaning toward weak spark...either coils or points/condensers. I would just get the hole points/condenser plate from cb750supply for $52 then see how it acts. Have seen several bad condensers cause very hard starting and bad idle.

 

[tommycarl]

If you need a manual you might try here for a free download: http://www.sohc4.net/cb750k-manuals/

 

[Primetime63]

Thanks for the advise! I do have a couple manuals now.. one is old Clymer and other is official honda..that's how I checked the gaps on the points and valves..but I think I'll replace the points plate anyways..I put all new spark plugs in recently but did notice the sparks didn't look that great..any advice on checking to see if the rectifier is shot still? I'm going to have my neighbor check my battery tomorrow. .also any help on uploading pics of bike on here? Trying to do it off my phone.

 

[tommycarl]

We haven't asked yet about the carbs and air filter... do you have a stock setup or pods? Maybe just velocity stacks?

You might try checking the insulators between the carbs and head to see if they're sucking air. With the bike idling as best it gets, try spraying around the insulators with WD40, starting fluid, an unlit propane torch... something so you can check to see if RPM increases when sprayed which would indicate that they're sucking air.

 

[dirtdigger]

Make sure the battery is good and fully charged before you do any checking of the charging system. The Honda manual has a decent check for checking all the components. First things first...what is the charging voltage when the bike is running with the RPMs at 3000rpm? If it is a little over 13volts then it is charging. Basically these bikes do not charge under 3000rpm, especially when the lights are on.

 

[Primetime63]

Alright. .I will have to check for leaks around the carb.. as for it..it is stock with stock air box. I replaced air filter along with oil when I bought bike a few months back...battery is on charger now..will check it hopefully tomorrow after work..I think this weekend I will pull off carb and get ready for clean/rebuild..never did this so wish me luck!

 

[Primetime63]

So I did a couple things..I charged my battery and read 12 volts..let it set overnight and checked again and still 12 volts..hooked it up to bike and with someone's advice gave it a good amount of gas and got it to start...check battery while it ran and was getting 12.7 volts...reved it up to 4-5000 rpm and was getting 13.5 volts. .I even unhooked battery and bike still ran....so I took it for about a 5 mile ride on highway and country road and when I got about a block from home the engine seemed like it wanted to die!! I down shifted and noticed a loss of power!!! Then when I pulled in the clutch to coast it died! !! I tried to restart bike and it wouldn't! ! Starter would just spin but nothing! ! Tried to kickstart and no luck!! So I pushed it home and checked battery. .it was reading between 10 and 11 volts..

 

[Primetime63]

Sorry for the long story! Also noticed like a clicking sound from engine while I pushed in neutral. .is this normal? I'm new to motorcycles and picked a bad one to start with I guess!! So would the engine dying be from carbs or charging system if you had to guess? I still plan on taking carbs off and cleaning but anything else to check now? Hopefully engine is ok....thanks for any advice

 

[dirtdigger]

How long did you crank it after it died? If your battery voltage was down that far then you either cranked it for a while or it is not charging all the time. With points they will still work on 10 volts. When it started to die did you pull the choke on to see if it would start to run again? You should probably also look to see if the valve on the tank has got a restriction causing it to run the carbs empty. After it died did you see if there was fuel in the carb bowls? Did yo check for spark when it died?

Clicking noise could be as simple as a dry chain or worn chain and sprockets.

 

[tommycarl]

I seriously doubt that after only 5 miles this could be the issue, but... if the vent in the gas cap is plugged, gas will stop flowing. I had that problem with one bike, but think it took 10-15 miles in my case.

I'm guessing that it has something to do with fuel supply, but checking the plugs is probably a good idea.

 

[dirtdigger]

I seriously doubt that after only 5 miles this could be the issue, but... if the vent in the gas cap is plugged, gas will stop flowing. I had that problem with one bike, but think it took 10-15 miles in my case.

You doubt what would be an issue?

 

[Primetime63]

I tried the choke and did crank it a little. .usually when it's warmed up it will start back up, but this time it seemed like nothing..I didn't check bowls cause I had to get kids from daycare..thankfully it dyed only a block from home!!

 

[Primetime63]

It was running fine, but then seemed like I was losing power..almost like I was running out of gas and the more gas I gave it the less power I got..until I squeezed in the clutch and it dyed...

 

[Marty]

That symptom seems to me as to be a overheating problem. Was the engine very hot. With the carbs not set up it may lead to a very lean running motor which can seize up as it gets hot. This could lead to all kinds of other problems. If you have no fuel flow issue, you have a carb mixture issue. I would also do a compression check to see what shape your rings and valves are in. If your riding and give it gas is there any smoke coming out of the tail pipe? What color is it? Pull the plugs and look to see if they are black or white or clean. Let us know what you find.

 

[Primetime63]

So this morning I pulled the carbs and now need to clean them..any suggestions on cleaner? I was going to soak them in something. .Recommendations? ? I took the plate off and removed the tops and bottoms of each carb..if I soak them can the floats be soaked as well? Also I noticed when I took the gas tank off and messed with the fuel valve it didn't pour out a lot when I opened it...was going to clean it too..so where should I start on the carbs? I've never pulled jets before..