Author Topic: Carb Sync - Low Pressure ?? (Read 6563 times)

Ericcb750 · « **on:** April 15, 2014, 08:03:52 AM »

Hey guys, First I want to thank everyone one here for their help in getting my first cb750 up and running. I finally have it where it will idle without any choke and doesn't die. This weekend I set the timing perfectly with the timing gun. I also used my carb sync gauges to sync them up. I have a question though. I got all 4 carbs sync'd up correctly to eachother but the pressure is really low on the gauges? I have the blue 4 round gauges (not the mercury type). I just not sure what this means or if this is a problem or not? I haven't had a chance to take the bike out yet on the road. I want to see if anyone has had this issue and if this is alright or if it's bad. On the gauge they're all reading about 3 vaccum inches?? Any advice would be appreciated. Thanks

strynboen · « **Reply #1 on:** April 15, 2014, 08:09:23 AM »

hey i use the same type..have you closed the restriktors schrevs.
..high or low..it just imp that the vacum are the same..not the value.
..this instruments are used on auto engines..and can do things we dont use..on mc..

high rpm gives higer resultats..i test at ca 2000/3000 rpm or more

Ericcb750 · « **Reply #2 on:** April 15, 2014, 08:44:54 AM »

Oh alright that makes sense. Thank you! I tested them at about 1200-1300 rpm. Maybe that is why they were so low?? Also I did use the little plastic valves and I had to close them a bit on all 4 as the needles were jumping all over the place at first. After closing them....steady as a surgeon . Thanks for the help. Is this RPM okay for the bike to be idling at ? 1200-1300 or so? If I put it around 1000 it seems to want to die out.

Duanob · « **Reply #3 on:** April 15, 2014, 08:49:23 AM »

That all sounds pretty normal. I set my idle as low as a can get away with when syncing. usually 1100 - 1200. Then set it around 1400 when on the road. And what strynboen says about just make sure they are all equal on the gauges thats what's important.

flybox1 · « **Reply #4 on:** April 15, 2014, 08:50:14 AM »

both my 350F and my 750K idled best around 1200-1250

Ericcb750 · « **Reply #5 on:** April 15, 2014, 09:01:10 AM »

Oh that's sweet! THank you guys, I was thinking " what the f did I do that it won't idle at 1000rpm?" It is pretty good around that 1200 -1400 rpm. I'm used to my Harley which thumps along when idling so to me the cb750 feels like it's racing at that speed. I think It's ready for the road test this weekend!!! I appreciate all your help guys.

TwoTired · « **Reply #6 on:** April 15, 2014, 09:26:37 AM »

If you don't have the stock induction and exhaust components, the book values for vacuum don't apply. If stock and in good condition, it should idle fine at 900 RPM.

Low tick over RPM will be effected by uneven cylinder compression, ignition timing equality, for the two coils, how clean the spark plugs are, and how recent and accurately the last 3000 mile tune up occurred.

The vacuum sync gauges read and average of the intake pulses. The damper valves simply reduce the meter needle swing by storing some of the peaks during the lows. The reading is most critical at low RPM, particularly if you seek a low tick over RPM.

The goal is to make each cylinder fire with the same vigor, only then will the engine idle smoothly at low RPM. Vac sync is only one factor though, which is why a recent and accurate tune up it highly recommended.

I don't buy the need for boosting the idle speed more than 100 rpm (due to it's age) from book values on the stock bike. Perhaps after some mods, that is an acceptable compromise. But, I'd want to know the reason why, rather than just accept such behavior.

Ericcb750 · « **Reply #7 on:** April 15, 2014, 09:52:05 AM »

The bike is stock, it has the stock 4-4 exhaust, stock airbox I put a new Emgo air filter into because a mouse had made a meal out of the other. Carbs are PD - 110 main, 35 pilot, screws are at 1.5 turns on each. Is it bad to idle it at that speed? If it is supposed to idle at 900rpm , what could be some reasons it doesnt? The bike has 17k original miles. when i bought i rebuilt the top end (valve seals, lapped valves, new pucks under the cam towers. cleaned up the ports (just polished). I rode the bike when I bought it from the PO and it rode great.

grcamna2 · « **Reply #8 on:** April 15, 2014, 10:07:07 AM »

Quote from: Ericcb750 on April 15, 2014, 09:52:05 AM

The bike is stock, it has the stock 4-4 exhaust, stock airbox I put a new Emgo air filter into because a mouse had made a meal out of the other. Carbs are PD - 110 main, 35 pilot, screws are at 1.5 turns on each. Is it bad to idle it at that speed? If it is supposed to idle at 900rpm , what could be some reasons it doesnt? The bike has 17k original miles. when i bought i rebuilt the top end (valve seals, lapped valves, new pucks under the cam towers. cleaned up the ports (just polished). I rode the bike when I bought it from the PO and it rode great.

Eric,
When you rebuilt it,did you do rings ? Have you done a leakdown/compression test lately ?

Ericcb750 · « **Reply #9 on:** April 15, 2014, 11:17:20 AM »

Hi grcamna, Unfortunately I did not do the rings.....Now that i'm looking back at it I should have so that I new they were fresh. Because it was my first one I thought it would be alright to leave them alone. I've learned a lot since then and I know now, I should have done them. I haven't done a comp or leakdown test. I'm not sure how to do a leakdown test though? I don't have a comp tester though, maybe can rent one at the auto zone or something? When I re-built the top of the engine I went from a new head gasket on up. everything below I left alone.....Darn

TwoTired · « **Reply #10 on:** April 15, 2014, 11:26:22 AM »

Quote from: Ericcb750 on April 15, 2014, 09:52:05 AM

Is it bad to idle it at that speed?

Depends on the temperature. It will run hotter at "fast idle". No big deal in the winter. But, in the heat of summer when no air is flowing past the fins, it runs hotter and cooks the oil faster. Marginal concern. You may just have to change the oil more frequently, if it becomes damaged.

Quote from: Ericcb750 on April 15, 2014, 09:52:05 AM

If it is supposed to idle at 900rpm, what could be some reasons it doesnt?

low compression, poor a/f mixture.

You should describe what happens when you attempt to idle that low.

I don't agree that you should have done the rings, assuming they were still functioning as they should be. No way they should be worn out at 17K if it has had proper maintenance and oil changes.

Read the FAQ about compression testing errors, if you decide to go that route.

Ericcb750 · « **Reply #11 on:** April 15, 2014, 12:10:56 PM »

Thanks Two Tired, I appreciate it. It could be the air fuel mixture. when I do turn the idle down low like that it sounds like the engine is working hard, I seem to be getting some noise from what sounds like the cam chain which I did tighten (i haven't run the bike after tightening it though) so this may help. I'm kind learning all this as I go so I do appreciate all the advice. I have a buddy who has a compression tester but somehow lost the 12mm adapter for the HOnda's.... The bike was taken care of very well and the guy I bought it from actually owned a shop in NY. he was the owner since the 80's (left all his insur. cards from then on in the pocket under the seat)

) so I know he was telling the truth on that one. I didn't think that 17K was too much so hopefully it's not the compression, I will see if I can borrow a tester from someone to check though? Hopefully I can find someone who has one around here.

Deltarider · « **Reply #12 on:** April 15, 2014, 12:49:01 PM »

Once I performed a sync very well indeed and was proud as a horse with two dicks I could make her idle between 800 and 900 no problem. I realised however an idle that low might jeopardize the oil supply in the upper regions and pressure. Personally I recommend an idle not below 1100, just to be on the safe side.

grcamna2 · « **Reply #13 on:** April 15, 2014, 01:09:07 PM »

It never hurts to document you're current compression for each cylinder on a given mileage/date;do what Too Tired said about checking the FAQ to be sure you're getting an accurate reading w/ a warm engine.

Duanob · « **Reply #14 on:** April 15, 2014, 02:00:45 PM »

Quote from: Deltarider on April 15, 2014, 12:49:01 PM

Once I performed a sync very well indeed and was proud as a horse with two dicks I could make her idle between 800 and 900 no problem. I realised however an idle that low might jeopardize the oil supply in the upper regions and pressure. Personally I recommend an idle not below 1100, just to be on the safe side.

I tend to agee. I'm not sure about PD carbs but the older carbs sound best at least 1100+. Even idling at 1500 doesn't make much difference in an engine that doesn't begin to make its power until well over 5000RPMs.

brewsky · « **Reply #15 on:** April 16, 2014, 04:50:33 AM »

I would not rely on the stock tach to read accurately at the idle end of the scale.

Before you worry about the correct rpm for idle, it would be good to know what the real reading actually is.

I gave my old analog dwell-tach away years ago........ (mistake!)

Bought a new digital tach, and it is completely useless

azuredesign · « **Reply #16 on:** April 16, 2014, 05:04:44 AM »

I don't know how important this is to you at this point, but my understanding is there is relatively little or no stator output at low turns. This is also one reason why I also like idle set to 1200 or 1250. Guage accuracy can be checked with an electronic guage, like a dwell meter if one must.

grcamna2 · « **Reply #17 on:** April 16, 2014, 05:48:38 AM »

I'l raise idle to about 1500 rpm to synch initially,then slowly drop it to finish off w/ fine tuning the pilot screw synch. if they want that.

MRieck · « **Reply #18 on:** April 16, 2014, 06:51:17 AM »

Quote from: Deltarider on April 15, 2014, 12:49:01 PM

Once I performed a sync very well indeed and was proud as a horse with two dicks I could make her idle between 800 and 900 no problem. I realised however an idle that low might jeopardize the oil supply in the upper regions and pressure. Personally I recommend an idle not below 1100, just to be on the safe side.

That is correct especially when the oil is hot and thin.

Deltarider · « **Reply #19 on:** April 16, 2014, 07:14:17 AM »

Quote

I gave my old analog dwell-tach away years ago........ (mistake!)
Bought a new digital tach, and it is completely useless

Yep, analogue is much easier to work with. I once saw a mechanic struggle with a digital emission tester. Hopeless and a digital tach is next to useless.
But... don't trust your onboard tachometer!

Ericcb750 · « **Reply #20 on:** April 16, 2014, 08:28:00 AM »

Hey everyone, Thank you all for the help and the advice, I really appreciate it. I do not have a separate tach so I am gauging things based off the one on the bike. That's good to know that it won't really hurt the engine if the idle is at the 1300 +/- where it is now. It sounds great when I rev the engine. I wish I knew some more people in the area with these types of bikes to have some help and see how their's sounds. I'm so excited to get this bike on the road, I can't wait to ride a bike older than I am!!! Cosmetically this thing is beautiful and I think I'm on the right track to get it running great. Don't need a beautiful 480lb paper weight. I hope to run into someone who knows about these bikes sometime so I can let them listen to it to make sure it's alright. I'll try looking at a tach also.

Also if the compression is a little low, will it help to put a little Mystery Oil or something in the plug holes to kind of lube the cylinder walls? Would that help the rings? As I said I don't know the Comp readings yet and will try and get that info soon. I'm just wondering if something like that may help if they are low? THanks guys, i appreciate it. Take it easy.

grcamna2 · « **Reply #21 on:** April 16, 2014, 08:52:50 AM »

That Marvel Mystery oil type stuff will help a little only if your pistons/rings have lots of carbon,etc. on them and that's the cause of compression loss.How did your firing zones look when you did the valve work ? lot's of carbon ?
I just thought it would be a good idea for you to at least know the compression of each cylinder w/ an accurate test.
I had done a few synch's on some bikes that had low vacuum numbers and it turned out their compression was low also. Eric,also while you're doing the carb. synch. make sure you have a Big fan blowing on high in front of the bike(I usually put it 1 ft. in front of it's front wheel) once it's fully warmed up so you don't cook the engine...a 1/2 hr. running like that is enough.

Ericcb750 · « **Reply #22 on:** April 16, 2014, 09:18:45 AM »

Thanks grcamna2, Actually when I opened the engine there wasn't a lot of carbon at all, a thin layer on the top of the pistons and the head around the valves and on the bottom of the valves. I cleaned it up with a little buff wheel on my dremel?? There wasn't much at all. I did clean the tops of the pistons off as well. I wish I had just gone through and done the rings but oh well, If I have to I will. I'm just hoping that I will at least be able to ride it this season and next winter I can pull the engine again. The other thing I was thinking too is that I did the sync test at 1200-1300 rpm, Maybe that is why it was low? Some others recommended doing the sync at at higher rpm ?? I did actually use a fan too in front of the engine, I was worried about it getting to hot, Thank you though, I really do appreciate all the help guys. I will see if I can get a compression tester at least so I know where I stand like you said, definitely not going to hurt. Maybe I should just fold and buy one??

grcamna2 · « **Reply #23 on:** April 16, 2014, 10:07:33 AM »

The rings should be Ok if oil was changed often as TT said.I purchased an adapter for my 12mm spark plug holes from Snap-On.I recommend you getting one from a Snap-On guy if you see a shop that's local to your area and you can hook up w/ the truck locally.I'd get the adapter/hose section first and then look for a gauge w/ a quick release that adapts/fits your Snap-On quick release.That size will fit Lot's of different bikes.

cheftuskey121 · « **Reply #24 on:** April 16, 2014, 10:18:36 AM »

Quote from: Deltarider on April 15, 2014, 12:49:01 PM

was proud as a horse with two dicks

This made me laugh out loud way more than it should have! I felt the same way when I synced my carbs for the first time, but yes I run mine at about 1100-1300 because it's hot as hell here in Georgia!

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Author Topic: Carb Sync - Low Pressure ?? (Read 6563 times)