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Thread: 1972 R75/5 Running Poor Acceleration

  1. #1
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    1972 R75/5 Running Poor Acceleration - SOLVED

    Greetings all, new member. Just did my newbie post in the new members section. Have a question about my '72 R75 and an issue with sluggish acceleration. I'm new to BMW (coming from Honda SOHC world) and i'm just getting the hang of it. Have gone through the bike overall to check things out, replacing missing or incorrect items here and there (clamps, plugs, etc.). Bike appears fairly unmolested but not much known about its history so i've done the maintenance items changed oil, air filter, trans fluid, drive shaft and final drive fluids. New petcocks and fuel lines along with tires.

    Bike has been cranking and idling well (I believe having NO idea how well one of these idles); however, its sluggish on acceleration especially at cruising speeds (around 60). I understand these bikes are not quickest or fastest around and i'm not looking for that. After warm up of ten minutes or so, the bike hesitates and shudders with any significant twist of the throttle. However, while twisting the throttle and reach down and put the choke at about halfway the bike immediately smooths out, accelerates very strongly and takes off like a champ. It will cruise like this perfectly until you of course come to a stop and then its idling in the 3k range. Pull off the choke, bike drops down to a few low and steady idle, but if you go to accelerate again there is sluggish response.

    I'm coming from the world of Honda SOHC's and at present in the dark about the Bings. While I get up to speed I took the bike to a local shop that I have relationship with, and coincidentally the owner rides a beautiful R90S. He gave the bike a once over for me and set the timing, rocker clearance and endplay and checked out the carbs. He said the diaphragms were good and passages all looked clean. He synced and dialed it and it is cranking and idling beautifully. He told me to try it out and see how it was running and bring it back if any issues and he'd sort it out without charging me again. He did not do a full clean of the carbs or a rebuild, as he was wanting to try the other things first just to make sure and save me some $$ on rebuild parts if not necessary.

    So, I'll be taking it back to him with the findings of the ride this past weekend, but I wanted to come to the well here to see if anyone had thoughts. I'm assuming it's probably needing a carb rebuild at this point? It just seems odd to me that its running generally so well otherwise.

    An additional odd thing regarding the throttle and i'll do my best to not butcher my description of this. There is a point towards the end of the rotation of the throttle that I continue to rotate the throttle the engine actually decreases in rpms or bogs down further. It's hard to tell if its just slowing back down or the mixture is going exponentially fubar that it bogs further. So, put another way, as I'm twisting the throttle the engine revs increase in step with the twist/opening of the throttle to a point near the end of travel for the throttle. But then as the throttle is opened beyond that point the revs fall off for those last maybe 10 degrees of rotation. I understand that's a horrible description. I'm not sure if this is problem with the cables or throttle assembly or if related to the overall poor running but i throw that out there too.

    Thank you for indulging the rookie questions.
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    Last edited by jcurt325; 01-17-2014 at 07:45 PM. Reason: SOLVED

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    First thing I would do is purchase the BING book (from Max's BMW) - I think it goes for around $12. It had detailed explanations of how the bings work, and it has several pages of Bing applications on BMW's that give you standard stock settings.

    Now, having said that, it sounds as if the carbs' mixtures are great for low end performance, but somewhere along the line, they sound like they are going lean (since choking seems to help). Personally, I would take them apart one at a time, and be sure not to mix parts between them. I would then think about replacing the Jet Needle, and the Needle jet. Also, simply make sure that the floats are floating and properly adjusted. Make sure they are actually shutting off the fuel flow as they are supposed to do.

    I also am wondering about the spark adjustment and then the spark advancing as it is supposed to. If it doesn't, it will be sluggish at higher speeds.

    I am not an expert, but this may help. There are certainly others who know more than I do.

    Other things I would check on the bike since it is new to you. BE SURE to remove and re-pack the wheel bearings. I was told my purchased bike was pretty well maintained. I found LOTS of things that weren't. But the most expensive and most dangerous were the front wheel bearings. DRY as a bone! I was lucky I checked them!

    Check and clean all battery connections. Also, if removing the front engine cover, be sure to disconnect battery first so you don't accidentally fry the diode board up there. Check and clean any electrical connections on the ignition and charging systems.
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

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    Beemerphan Radar41's Avatar
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    Cool R75/5 Running

    My suggestion if you are new to Airheads, would be go to Snowbum's site and do some reading.

    It is an excellent source of information.

    http://bmwmotorcycletech.info/techni...icles-list.htm

    The suggestion of getting the Bing book should be taken. It is also available if you call Bing.

    Another suggestion is to join airheads group.

    http://www.airheads.org

    There are many of us around the country/world who have rebuilt these wonderful machines.

    Your profile doesn't say where you are??? There are no doubt some local Airheads who would be glad to lend a hand to get your machine sorted out.
    Last edited by Radar41; 01-14-2014 at 06:02 AM.
    Don "Radar" Wreyford
    00 K1200LT, 98 R1100GS AE, 84 R100RS, 76 R75/6 (rebuild complete)
    MOA # 91738, RA #27032 , ABC #7915 - "Insanity is doing the same thing over and over again, but expecting different results" - Albert Einstein

  4. #4
    Registered User godfather's Avatar
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    First off, congrats on the airhead and welcome to the MOA jcurt325?

    I am relatively new myself but have learned the very last thing to adjust or mess with is the carbs. Re check valve lash and timing. Check to be sure the air intake is not plugged up. When you replaced the air filter did you check to see if there were any obstructions (say mouse nets) under the starter cover and in the air suction hood? Check spark plugs to be sure they're still clean and check that the throttle cables have proper free play too.

    Good luck and keep us posted.
    Attitude is everything!
    1978 R100/7
    08' V-Strom 650 great light weight tourer

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    When I heard the bogging down at 60 or so, my first thought was diaphragms. But if they've been checked and inserted into their proper detents in the top of the carb body, then maybe they're OK.

    I like the idea of the floats and the levels. For these 32mm carbs, IIRC the height of the fuel when measured in the center of the bowl is around 24mm. This is easily checked by letting the bowls fill up, turning off the petcocks, and carefully removing the bale wire holding the bowl on. Then measure the fuel. If running lean (less than 24mm of fuel), you could be bogging down at speed, especially when rotating to wide open throttle...lots of air, no gas.

    You could also have a misadjusted throttle cam. When the top of the throttle housing is carefully removed, you will see a ring of teeth on the grip side and a horizontal ring of teeth connected to a small chain. There are marks on both sets of teeth that must be aligned at zero rotation of the throttle. It could be that you have a misadjustment there...worth checking. Also worth putting a small amount of silicone lubricant on the meshing teeth.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    #4869 DennisDarrow's Avatar
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    Diaphragms are fairly cheap and I really go along with the idea that that is the problem........If it was me, instead of just checking them I would get a new set along with the rubber "O" rings for the jets and install them. There can be rips or tears in those thin rubber diaphragms that one doesnt see........ Won't take but a minute and what you desribe about falling off when going 60 or so and then catching up.....pretty much gutless with roll on......that is classic diaphragms.

    I agree with Jimmy about checking the advance mechanism. Take it off and clean it and then grease it with the PROPER grease. Bosch Ft 1 v 4 Grease......A tube will last for your life..........

    HUGE NOTE..............When removing that front cover to get into the points.....ALWAYS......take off the ground cable back at the transmission before removing that cover...........ALWAYS........If you don't we will soon be getting a thread about burning up the diode board and how to fix it............God bless......Dennis

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    Quote Originally Posted by 20774 View Post
    , IIRC the height of the fuel when measured in the center of the bowl ...
    I have been racking my brain, but what does "IIRC" mean? I understood your sentence and overall meaning, which was great, but can't figure out what IIRC means.
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

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    Quote Originally Posted by jimmylee View Post
    I have been racking my brain, but what does "IIRC" mean? I understood your sentence and overall meaning, which was great, but can't figure out what IIRC means.
    Several meanings generally speaking; however, I would venture to guess in this instance it is used as a acronym for IF I RECALL CORRECTLY......
    Hope that helps.

  9. #9
    Administrator 20774's Avatar
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    Correct on the meaning of IIRC...I didn't want to go look it up but knew the number was in that range!! It's the second entry on this web search:

    http://lmgtfy.com/?q=IIRC

    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    Quote Originally Posted by 20774 View Post
    Correct on the meaning of IIRC...I didn't want to go look it up but knew the number was in that range!! It's the second entry on this web search:

    http://lmgtfy.com/?q=IIRC

    Sorry abut that, but I don't do phone texting so I am not aware of many of those sorts codes! lol

    That may be the only one. Oops, I forgot IMHO
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

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    Thanks for all the pointers! I've heard the point about the battery and the front cover from a coupe of others who passed along their "unfortunate" wisdom. Bing book ordered Jimmylee. Will let everyone know how it progresses. Thanks so much.

    Curt

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    Also, Radar41 - I updated my profile to add location. I'm in Savannah, GA. Thanks for reminding me, still getting settled in. Thanks all.

  13. #13
    Beemerphan Radar41's Avatar
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    Cool Airhead Help

    Quote Originally Posted by jcurt325 View Post
    Also, Radar41 - I updated my profile to add location. I'm in Savannah, GA. Thanks for reminding me, still getting settled in. Thanks all.
    thanks,

    Nathan Mende is the Georgia Airmarshal and can put you in touch with some serious Airhead help .

    Sent a PM with his contact info.

    radar
    Don "Radar" Wreyford
    00 K1200LT, 98 R1100GS AE, 84 R100RS, 76 R75/6 (rebuild complete)
    MOA # 91738, RA #27032 , ABC #7915 - "Insanity is doing the same thing over and over again, but expecting different results" - Albert Einstein

  14. #14
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    And its ANSWER time!

    Going through all the great suggestions....check the carbs, look for obstructions in air path. Air filter clear - check. Under the starter cover clear -check. Air intake scoop clear....wait, so clear in fact its missing and there's a giant hole in the top of the cover where it should be. Never got to the point of removing the tank to visualize it before. Previous owner didn't mention it being lost. So based on everyone's suggestions of a lean condition at speed and the fact that the enricher balances things back out, needed to reduce the hole and the air coming in and see what changes. Not having a replacement scoop on hand, couple strips of duct tape over the cover to reduce the opening for a test. And the thing runs like a scalded dog! Flies through gears. Needs no choke. Smooth and steady. Quick run over to Ebay and picked up a replacement intake scoop (and starter cover with all the clips for the scoop) for $25! I would think even in this test setup its still probably getting a little more air forced in than usual since the scoop is rearward facing and at present its getting pushed under the tank at speed. We shall see how it does with the scoop back in but i'm thinking we can call this one resolved!

    Also, the issue of the throttle falling off at full open seems resolved as well by this, though full throttle is now not needed.

    Thanks again everyone!

  15. #15
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    The intake scoop (and screen if one existed) shouldn't really restrict the air coming into the filter. It's pointed backwards just that it doesn't actually scoop hard particles directly into the top of the engine...theoretically, a hard particle will have too much momentum going backwards to make the 180 degree-ish turn back into the scoop.

    I think you're experience has just demonstrated there's something wrong with the air-fuel mixture settings in the carb. But physically restricting the amount of air that could get to the carbs, you returned the balance to a reasonable number.

    But certainly, go ahead and put the scoop on and see what happens...report back on that and let's see if that's the answer or not!
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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