Author Topic: 78 carb and seat question... (Read 3397 times)

xntrik · « **on:** September 29, 2014, 02:43:48 PM »

Hello all, I'm nearly finished and hope to have my 78 done enough to get inspected next weekend. Though I have a couple things to cover first.

Bike is stock....exhaust, jets, air filter (old but clean), mix screws at manual settings, bench synchronized, floats set perfect.

The bike will start with with just a bump at full choke and idle all day after it warms for a minute. After that, an easy roll of the throttle will kill it unless the choke is still full on, or at least 1/2 to 3/4.

After its sat and idled for about 10 minutes, I can ride it and again, have great full range acceleration with choke pulled. After about a mile or so of actual riding, I can push choke in and although I have hesitation/stumbling up o 1/4 throttle, once past that, it screams like a champ. I believe I may have a slight vac leak at the #2 carb insulator at the carb itself, and plan on pulling carbs tonight to verify/fix.

The accelerator pump is only squirting (only a dribble actually) out of the #2 carb, and I verified all passages were clear/clean with no holes in the diaphragm before previous reassembly.(also reading that this should not effect a slow twist of throttle though). I plan on addressing this another day, as it looks like that can be replaced with the carbs on the bike.

When I rebuilt the carbs, I was a stooge and didn't check the needle position. I can't recall where, but I believe I read the 78's needles are not adjustable. Is this true? If not, is there any way to verify needle setting without splitting the rack?

I know my insulators are crack free, though stiff. And hope some new clamps I got will cure the #2 slight leak issue.

Aside from addressing the leak, double checking needle setting (which I really hope I don't have to split carbs to do), and fiddling with mix screws, is there anywhere else I should be looking?

Also, I need a seat. I'd like to have a stocker, but anything will do at this point. What seats will bolt and go to the 78 with the latch lining up? I'm reading conflicting things on wether they're interchangeable between years.... Of course if Someone has one they'd part with, please let me know.

Thanks!

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Bokeh · « **Reply #1 on:** September 30, 2014, 12:21:20 PM »

For the Carb boots - heck out this thread. I have not tried it myself but it looks promising to soften up the rubber.

http://forums.sohc4.net/index.php?topic=29707.0

Also, I would expect the Seats should fit between the years. I have a 76 that came on my 77. The difference between those two is that the 76 has the Rubberized grab bar that mounts to the seat. So if you want it to be accurate for the year you may need to get the exact year. The pans seem to be the same. I cannot verify the same for the 78, but the frames should be the same between the 77 and 78 so the mounts should line up. Again as I stated I can only verify that the 76 fits on a 77 frame for sure. I have seen some pic of what looks to be early 80's seats on the Matic, but again I cannot verify the fitment.

Im in Wentzville, MO about 45 min. west of St. Louis if you are anywhere close. I may have an extra pan. PM me if you like

xntrik · « **Reply #2 on:** September 30, 2014, 11:17:17 PM »

Bokeh, thanks for the response! Looking a little closer the #2 doesn't look like its seated all the way against the carb... Dead ringer for a vac leak. I'm planning on pulling it apart in just a bit to see if I can check needle position with just the caps off, and perhaps try to source some local supply for wintergreen to soften those isolaters up before re-assembly. I can't seem to figure out how to PM about that pan, but I'm trying.

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martin99 · « **Reply #3 on:** October 02, 2014, 02:22:54 PM »

Sooooo...

Firstly, there's more to check in the accelerator pump circuit than just the diaphram, although ensuring there are no splits or holes is a good place to start. Did you make sure the holes in the Mickey Mouse ears on the diaphram were clear? If not, you will have restricted fuel flow. There are also two checkball valves in the float bowl that you need to verify are working. Also, there is a small o-ring between the float bowl and body which if flattened, distorted or the wrong size will compress too much and cover the passageway when the bowl is attached and again may cause restricted fuel flow. When you twist the throttle you should see a good strong squirt of fuel coming from the brass posts into the inlet tracts.

The rubber tubes linking the carbs have little brass restricters in them. These are often overlooked and may be partially blocked.

Hesitation/stumbling up to 1/4 open suggests pilot circuit to me. Have you pulled the pilots and cleaned them? They come out with a tug and slight twist with a pair of grips. It's not really possible to clean them properly unless you remove them. Doing this had a marked effect on my bike at idle.

78 carbs will likely be PD42s, which don't have adjustable needles. PD41s do have adjustable needles. Check the number stamped on the side. As you say it runs like a champ over 1/4, I wouldn't expect there to be a problem there. Beware of removing the needles with the carbs in situ - it can be done but you need the dexterity of a neuro-surgeon and if you drop one of those tiny washers or springs you'll kick yourself.

Sounds like you're close - good luck and enjoy your ride!

EDIT - Just realised I'm posting in the 750A section, which I know nothing about. Doh! If the above appears relevant, then great, if not just ignore me!

xntrik · « **Reply #4 on:** October 03, 2014, 11:42:23 PM »

Perfectly fine oldskool. I'm sure you're dead on with the accel pump, I think the Mickey ears are swelled shut. I remember them looking pretty swooshed when I cleaned the carbs, and pretty sure I remember checking the one-way function of those passages.

I ended up pulling all boots and found to my horror what every "A" owner dreads.... My boots are trashed pretty good at the head side. Looks like I'll be building a set or scrounging hither and fro to piece one together.

In the meantime I was able to seal them up temporarily, but have the carbs acting up even worse than before. I'm going to try a sync and see what that doesn't do. Previously they were just bench synced.

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raymond10078 · « **Reply #5 on:** October 04, 2014, 10:06:38 AM »

Did you remove and clean the emulsion tubes?

xntrik · « **Reply #6 on:** October 04, 2014, 10:31:45 AM »

Yes....everything was cleaned. I synched the carbs last night, and verified I sealed up the isolators. Now it'll start with half choke, and after it warms up will idle with no choke... Blip throttle with no choke and it falls on its face.

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xntrik · « **Reply #7 on:** October 06, 2014, 08:57:45 PM »

Fiddled with idle screws last night, made a little difference, but not much. After it was thoroughly warmed up, with screws 3 or 3.5 turns out, rode it fine -stop and go with no choke- for about 3 miles. Then right back to the same old same old. I'm going to pull these stubborn ba$tard$ apart one more time. Triple check EVERYTHING, and buy an ultrasonic cleaner to have at them. I'm even going to mic the needles, idle screws, etc.... If this doesn't work, I'm swapping for K carbs. I really want to keep it orig. But if I'm spending 130+ on full rebuild kits, may as well sink that into a diff set that everyone is saying makes it run better anyway.

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Bokeh · « **Reply #8 on:** October 07, 2014, 08:28:11 PM »

Gotta question. The main needle in the slider - I have a set I'm rebuilding and the numbers on the needle show 048 stamped on the side. The rebuild kits 065 stamped on the needles. Are these numbers the correct size and if so which should I start out with? BTW met xntrik over the weekend. Great guy, having a #$%* with his carbs. Number 3&4 running rich at least from checking temps. #1 running cooler and #2 running the coolest about 60 degrees cooler than 3/4. He tried blowing propane etc around boots and all around looking for leaks so this has to be in the carbs right. Also, I wanted to verify I didn't tell him anything wrong - When engaging the choke on this bike it actually enriches the mixture not cuts it out correct?

xntrik · « **Reply #9 on:** October 07, 2014, 08:42:46 PM »

Adding to that..... What are the needle size variations through the years?

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xntrik · « **Reply #10 on:** October 09, 2014, 06:52:16 PM »

Got to really eyeballing my exhaust tonight, only to find it's pretty messed up. I had noticed the right side was usually a little louder than the left, and thought maybe they'd just need re-packed. Come to find out this is -what I think- a MAC system. After pulling the baffles, found that they're totally different. One looks like a MAC baffle, the other is some cobbled together welded deal. So now I need an exhaust too (when baffles are 50-60 a piece, may as well just get a used replacement). And I find this out a day after Mr. Freeze sold his, DAMNIT!

Anyway, with baffles pulled, exhaust temps are same between 1,3,4 now. 2 is still 40-50deg cooler.

I'm using a digger machine tonight to double check for vac leaks in carbs/rubbers, then pulling carbs and totally going through them again.

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xntrik · « **Reply #11 on:** October 09, 2014, 06:52:51 PM »

Fogger machine.

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xntrik · « **Reply #12 on:** October 09, 2014, 11:11:49 PM »

1-3cyl's @ 140psi, 4@148-150psi.

Fogged all cabs while attached. Only leaks are at throttle and choke shafts. I replaced what was left of the felts (only a couple were left) with o-rings that seemed to seal up pretty good. At least they caused some good drag on the shafts without seriously impeding their operation. No leaks from boots.

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xntrik · « **Reply #13 on:** October 10, 2014, 12:06:41 PM »

Who's got two thumbs and's goin round tops? That's right... THIS GUY!

Just strapped on a set of K4 round tops borrowed from a buddy. Old, grimey, haven't been run in 8 months round tops. Fired right up! After a brief warmup, good throttle response, no stall under load. Trolling eBay as we speak. Screw those junk PD's! I'm tired of screwing with them.

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Bokeh · « **Reply #14 on:** October 11, 2014, 10:32:06 AM »

Great news!

dave the welder · « **Reply #15 on:** October 12, 2014, 02:13:52 PM »

welcome to my world Round Tops Rule

J-Dawg · « **Reply #16 on:** October 12, 2014, 07:11:58 PM »

Seatwise...I have a stock 77 hondamatic seat if interested...bolts to hinge and lined up with my latch no Issues. Only 2 tears at stock seams, easy fix. If interested. PM me

xntrik · « **Reply #17 on:** October 12, 2014, 08:59:09 PM »

Jdawg, thanks so much! But I already procured one.

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xntrik · « **Reply #18 on:** January 13, 2015, 02:42:29 AM »

To bring this back for just a quick question........ Which way do the slides face. Domed cutaway toward head, or filter.... Pretty sure they face toward cylinder head right? I'm trying one LAST time to clean the pi$$ out of the original carbs. I'm a dip and didn't take a photo on tear down....

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Bokeh · « **Reply #19 on:** January 14, 2015, 08:31:44 AM »

Cutaway towards the filters/butterflys.

News:

Author Topic: 78 carb and seat question... (Read 3397 times)